Quoted from yet another site:
Underdrive Pulleys Our Testing
by James Walker, Jr. of scR motorsports
"In general, the other pulleys in the system are not sensitive to weight like the crankshaft pulley, so harmonic imbalance is not a concern here.
If you do decide to underdrive, though, be aware that while there may be a slight horsepower gain (read more on this below), your components will not be operating in the ranges determined by the factory.
If you are on a race track at constant high RPM, this may actually prolong the life of these components, but on the street, it could go either way a water pump or power steering pump which is designed to operate at a given speed may not be as efficient as necessary when underdriven.
So, in light of the fact that 3 HP was completely without a belt, we estimated that the best we could do was to underdrive a few components by 20% - or a theoretical 0.6 HP gain (3 HP x 0.2). After that, it hardly seemed worth it to try to custom fabricate pulleys for less than a 1 HP gain.
I know, I know stock pulleys, like stock suspension bushings, arent sexy, and you cant brag about them in the pits|but in racing horsepower talks and marketing BS walks. We chalked up this exercise as yet another learning experience in the quest for REAL horsepower. Maybe someday well find some.