I certainly agree with Scottly about the small Horsepower drain with the stock alternator.
The only saving grace would be the weight of the rotating mass. Less weight in the flywheel effect does take less horsepower to change its inertial momentum.
I would suspect the most difference would be on a bike with minimum weight, that is just waiting for the engine RPM to pick up.
Seems like a waste for a street bike. I can see it for racing/drag racing, etc. where people will trade money for extra margins in speed in tenths of a second.
Also note, that less flywheel can make idle speed, uh, less smooth and taking off from a stop require more throttle twist, as a lightened flywheel helps less at getting the bike to more forward smoothly. Again, racers probably don't care about that.
Cheers,