Hard race braking probably uses a lot more power than it puts back into the batteries, or more gets transformed into heat...

But what about slow, easy braking? Slamming on the brakes and releasing right away isn't going to put much usable energy anywhere but heat... That could be why there's longer run times on a track with mechanical braking.

Maybe someone with a data logger can do some tests, and try it with moderate braking instead of panic stops?