10 years ago I started riding again after MANY years away. On a test ride, I put max power into the pedals and my foot slipped off. As we all know, the crank comes around and the pedal wacks you in the back of the leg. It took better than 6 months for the bruise to go away. That's what made me investigate clipless.

So, my guess is that there is no max power advantage, but I suspect there may be an improvement in consistency.

My progression has been:

1) Ritchey ATB pedals
2) Crank Bros. Quattros
3) BeBep pedals (current fave!)