^^^ This is the right idea.
Addimng my vote for you to rent or borrow a road bike and test it vs just wondering. Personally, I ride my gravel bike on group rides without any concern of being dropped. But the road bike is indeed snappier for closing gaps.
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Well, I hope you aren’t that much slower as I have almost the same set up (Corsa NEXT 34s and Zipp 303S wheels)! I don’t feel it is meaningfully slower than my endurance road bike (which is far from an aero bike), but I am not setting a sort of speed record. I agree with others that the fact this fits you well matters a lot (including the ability to ride in the drops for longer periods) and you could eke out a bit more speed with GP5000s and certainly gain some free watts with the right clothing.
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Excellent idea! Honestly never occurred to me 
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You’ve already swapped the wheels and tires so that’s the biggest difference between a road and gravel bike taken care of. Body position and clothing can have an impact to aerodynamics too.
The biggest effect to riding in a group is how efficenlty you can ride in the draft of the rest of the group and save energy.
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The difference is not going to be massive, but it’s not nothing either. For three seasons, I used a first gen Cervelo Aspero for both road and gravel, swapping two wheelsets. I didn’t have any particular problem keeping up with group rides, but did have the general sense I was leaving some speed on the table. Two years ago I purchased an SL8, it’s pretty tricked out and weighs about 14.5 pounds, it was definitely a complete splurge bike, which I hadn’t done in a really long time. Was looking for a balance of aero for general riding, and lightweight since the only races I ever do are hill climb TTs. Cervelo was about 17.5 pounds in road mode. Initially I used the same wheelset, riding position was the same but with 2cm narrower handlebars.
After a few weeks of riding I ran a comparison of similar rides under similar conditions on gently rolling terrain, and came up with an average advantage of roughly 14 watts for the SL8. That translated into about three-quarters of a mile an hour faster on brisk solo rides. A little bit of that was the narrower handlebars, but the bike was most of it. The SL8 feels significantly faster, feels lighter and snappier out of the saddle. It dives into turns more easily, but takes more attention to hold a straight line, which took some getting used to. Aspero is slightly more forgiving on large bumps but SL8 is comfortable enough, I’ve done long rides of up to 130 miles without a problem on the SL8. Overall the SL8 is more fun, but I would say that the Aspero does feel a bit more solid on very fast downhills.
That said, while the subjective difference to me was quite substantial, it’s not a complete game changer either. Folks here have given some good suggestions for how to maximize performance with your current ride. Using a gravel bike on the road does involve some compromises on gearing, handlebar width and pedals, depending on your setup, and those may or may not matter to you. And there are many road bikes you could purchase that would have significantly less of an advantage than my SL8; by the same token there are bikes that will give a larger aero advantage as well.
Bottom line is that you can problably be just fine on the road with your existing bike, but if you are really serious about road riding there are advantages in having a dedicated road bike; at a significant cost of course!
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I promise you the Tarmac is not 14 watts more efficient with the same wheels, tires, and bar width.
With that said, Tarmacs feel great and I bet you give it more beans when riding the Tarmac than the Aspero. It just feels good to push the pedals on a race bike…
It had the same wheels and tires, same position other than 2cm of handlebar width, same brand of power meter (Power2Max). I eventually got new wheels so I only compared my early rides when riding with the same wheels (although they didn’t make much difference). The width might account for a little bit of the difference, and it’s a ballpark figure based on outside rides rather than structured aero testing, so it’s not exact, but that is in line with the differences Tour Magazine tests show from a decently semi-aero frame such as the SL8 to non-aero frames. Very clear pattern of the SL8 being faster in my power & speed data, but then again keep in mind I’m not claiming 2-3 mph gain or anything.
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I don’t doubt the Tarmac is faster but the recent EC article on incremental gains puts the total drag of the frame at 25 watts.
If the Tarmac is indeed 14 watts faster, most of that is coming from rider position and bar width.
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Well, first of all a single watt figure for frame drag is unrealistic as that will vary greatly depending on the speed. There’s a chart on weight weenies that lists the Tour Magazine bike tests and the total wattages range from 199 watts to 234 watts, so a max difference between bikes of 35 watts. That does include wheels so might overstate the differences a bit between frames, but obviously those differences would be impossible if the typical frame drag is around 25 watts.
The difference in aero performance between my two bikes also includes hidden cables and a much more aero handlebar. And I can say with confidence the only difference in position is the 2cm wider handlebar, which might account for a few watts of the difference.
Now, if you really want an outlandish claim, Cervelo claims their latest Aspero-5 aero gravel bike is 34 watts faster than my OG Aspero. I don’t believe that figure for a moment, but I don’t find it hard to believe that new bike is a lot faster than my old Aspero.
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34 watts is utter BS. Drag scales (non-linearly) with velocity so that difference is probably true at some velocity but not one you or I are riding at…
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Sorry if someone has already said this, but ensuring you have an optimised fit is likely the biggest gain (assuming you are not already optimised), and will be the same regardless of the bike.
A bad fit on a faster bike = slower rider.
So, is the crank length right, is the saddle height right, is the bar width right, etc. in order to allow you to get the most power transferred to the pedals in the most efficient way and in a position that in comfortable so that it can be maintained.
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I have both a Revolt and a TCR. The Revolt is set up for mixed surface riding. I selected the Revolt because it has excellent road manners. My first gravel build felt a bit like I was towing a trailer when riding paved segments. I haven’t experimented with optimizing the Revolt for road use but have ridden the Revolt in groups and on roads that I typically use my TCR for. On flat segments in a pack the Revolt does well. Just don’t ask me to sprint or take a pull. Once the road pitches up for any distance I’m off the back (more than usual). As the ride progresses I can tell I’m doing much more work to keep up than on the TCR.
So what are my thoughts on the performance difference? First weight and geometry. The Revolt is heavier, less aero and a little more robust. Gearing is GRX, TCR is Ultegra. Tires on the Revolt are 40cm Maxis Reavers, The TCR, GP5000s.
If I had to adapt the Revolt to road I think narrower tires with smoother tread on lighter wheels would make the biggest difference. The plus is that you would still preserve the unpaved capabilities of the bike with a wheel change for those rides. The drivetrain would be next, but that would affect unpaved climbing and the occasional single track I do on the bike. There isn’t much you can do about the bike weight or aerodynamics. A fit to be more aero might help. But you say the bike is comfortable so messing with fit may have some downsides on a long ride. Clothing, helmet, bottle cages, the amount of stuff in your saddle bag will probably bring marginal gains.
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We’ve sold several Revolts to new riders who wanted a ‘do-everything’ bike, but ended up using them solely as road bikes. I’ve also used a Revolt as a road bike from time to time. Those SLR2 wheels aren’t fancy but they are very good, and you put a set of road tires on, so you’ve done the sensible gravel-to-road transmorgrifications.
The difference in effort when rolling along, even at high speeds, is negligible if you’re in a group. Even when riding solo or up front, you’re not going to see a 2 mph or 20 watt difference between a gravel bike set up for road and an actual road bike.
Where you will feel the difference is in the handling and playfulness of the bike, as well as in the way the bike feels in specific situations, such as standing up to maintain momentum and hammer over a roller, or sprinting, or bridging a gap. In such situations, the nice svelte road bike might only be 2% more efficient than the road/gravel bike, but it will feel five-hundred-million percent more efficient (no exaggeration), and a heck of a lot more fun.
Oh, and regarding the gearing, that gravel gearing might work on fast flat rides, but you’ll get plenty of R&R descending time if you ever ride in the mountains.
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There’s always a tradeoff. There are sections of road where my road bike says “come play!” in a way that the gravel bike never does. There are also sections of bad pavement where the gravel bike’s stability is much more comfortable(and comforting) than the road bike’s responsiveness.
If I had to be n=1 with a gravel bike and two sets of wheels, I think I’d still come home from rides smiling. 
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Others have hinted at this but not said it outright. Where are you in the group as you go through this ride?
You say you are keeping up, but does that mean you stay in the protected area in the back of the group? Or are you taking pulls at the front and you’re one of the ride leaders? Anything to do with aero is almost irrelevant if you are solidly in the draft, and the larger the group is and the more riders abreast the more irrelevant aero becomes as the savings from being in a big draft are much larger than anything you can actually change. Someone who is getting dropped out of the draft on hills for example is unlikely to be able to fix that with aerodynamics.
If you are pulling on the front, in a breakaway, getting dropped and then hammering to catch back on, bridging a gap, etc.. that is when your aerodynamic profile matters.
If you just replace the bike that makes the least difference if your fit stays the same. If you can improve your fit aerodynamically that’s the largest gain. Likewise if you can improve your ability to hold more aerodynamic positions without changing your bike at all that is a very large improvement. This is where spending money on bike fit and cross training, working flexibility, etc.. can have huge dividends. Changing handlebars, changing to a bike frame that lets you comfortably get a more aerodynamic position, changing cranks in a way that lets you unlock a better position, etc.. are all your biggest gains compared to a different frame shape in the same geometry/fit. If you’ve already got fast tires and your drivetrain is in good shape and your bike isn’t super heavy you’re probably good there.
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For those still following this topic, I recently did a post on this topic where I calculated the losses I had by using my gravel bike for a road ride: https://www.instagram.com/bicycle.engineering/p/DVsduFYiKuX/.
The difference was a quite significant 22.8 W (203.3 W vs. 180.5 W) in normalized power. However, most of the difference would be gone with a change to road tires. And if I switched to the same cockpit as on the road bike, it would be down to just 0.4 W. This was on a solo ride, the difference would be even smaller if I was drafting in a group.
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While I generally agree that a gravel bike with road tires won’t hold you back in a road group ride (assuming the bike is set up appropriately) I do notice that drafting very closely doesn’t feel quite as easy on gravel bike. I think it’s due to the feeling of the front wheel being significantly further ahead of the bottom bracket, and thus my weight is less forward and my body is actually further away from the body of the rider in front of me. It’s marginal, but it’s one of those mental things that keeps me using my road bike for group rides.
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