I tried to test this in the “aero sensors dummies” topic here but my data was not the best that day. I basically did some out/backs when playing with the adjustable angle. Anything in the “reasonable” range of 10 - 25 deg didn’t seem to register any difference.
The key to the whole thing is how the angle interacts with your reach and pad width and where your head goes with the angle you use. In general, I just find the angle helps you hold still with your head down/tucked without feeling like fighting to hang on.
I’m at 18deg. I was going for 20 on the adjustables and 18 was “good enough”.
It’s less about the angle and what the angle does to the shape of your body. It’s highly variable. In my testing of athletes not much changes until you hit 25 with a forearm angle. But again, it’s very rider dependent. I’ve seen it make people slower, faster and neutral. The big thing is to test all or as many variables as possible the. Find the one that you are most comfortable in.
Most of my aero stuff is track focused these days, but I also have my TT bike setup as close as possible without having to buy a second set of Wattshop extensions. I can ride comfortably in that position.
I think one of the biggest benefits is we’re no longer slamming our aerobars 15-18cm below the saddle. The drop on mine is like 8 at this point