^^^ 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 
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.
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.
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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