It is not 100% but the area we do use a power meter is for testing aero stuff. We have not yet done it at the velodrome, but have doing repeated runs on the same course at the same power while also looking at HR. I've seen HR go up as more core is used to stabilize higher profile wheels for example. It still ends up being a guess, but it can be an educated one and many tests are repeatable when run a month later. I can't pick up cable tie differences, but helmets, wheel profiles do tend to show.
Where this becomes a bit more complicated is choosing a more aero position that also is less powerful. So getting a more acute torso/hip angle while more aero holding the same watts, but the max watts in that position are also lower. That is where/why we used an experienced power guy for bike fitting - even though power is not used for training.