I ride mostly older stuff, not as an Eroica purist, but because I don’t race, built out some of my stable and parts bin as a poor graduate student, generally dislike consumerism and am okay eschewing some of the diminishing-marginal-returns-for-cost game. But I am interested to quantify some of the gains I’m leaving on the table.
(For the record, I also own a mountain bike with a slack HA and hydraulic disc brakes. My second and third times doing Eroica rides I used freehub, newer-style aero brake levers, and bar-end shifters that hadn’t been set to friction mode.)
For awhile I’ve been meaning to look into what-advantages-I’m-missing-out-on with road pedals. I started out on Look delta cleats in high school, and ended up accumulating plenty of that pedal system. I use red (9-deg float) cleats. Thanks to the summary on the most recent Geek Warning, here’s what I’m looking at:
Stack Height
Less difference than I expected. Based on a quick Google AI query, Look Delta system’s stack height is barely more than SPD-SL (non-Dura-Ace tier) and Look Keo (non-Blade series). I’m skeptical that stack height has significant pedaling-efficiency gains, and the differences here are small enough that I’d be surprised if there’s a measurable effect on biomechanical tracking from stack height itself.
Significant stack height gains can have aero impact by lowering the rider, but I’m not very aero on the bike anyway. Maybe there’s more to gain by cornering clearance, but I’m mostly riding the frames (and BB heights) that I already have, usually with wider tires, albeit wider-stance with pedal washers and also 180mm cranks. But I’m not maxing my pedaling-through-corners anyway.
Cleat durability
SPD-SL cleats are designed to be more durable (and less slippy) while walking than the only-plastic red Look cleats I use. I’ve always carried cleat covers on rides for my safety and cleat durability when walking around. A bit more pfaff, but my cleats are plenty durable.
System stability
It’s harder to find clear data on this, and I’d imagine Look Delta cleats/pedals have less precise interface, but it’s also dramatically more stable than my walkable-recessed-SPD option.
Weight
Some difference, and matters more b/c it’s rotating weight, but in the context of big legs and size-13 shoes, pedals are a small component.
Pedal Durability
SPD-SL has a great track record, but as mentioned above, I have plenty of spare Look Delta system pedals, and the tool to remove the bearings and clean/regrease.
Conclusion
It seems that the gains from SPD-SL are more marginal than I’d expected and/or the Look Delta system held up better than I expected. The biggest practical thing is not having to carry cleat covers. Am I missing some important factor here?
Or, should I accumulate a bunch of SPD-SL pedals now that the bleeding-edge folks are gonna be chasing that extra 2.3mm reduction in stack height? 
