well if its harmonicly balance with the crank wouldnt a new flywheel **** it up too? if ne think im going to say that less weight on the cranks (in the way of flywheels and pullies) is going to be better on the barrings of the engine. as long as its not a hack product that is not balance properly.

as for underdrive... im not a fan. since i have a b16 with just the altentor being driven im more into just having the single pully ,that i believe unorthedox makes, rather then the extra weight from the power-steering and AC.

its a general rule that the lighter you are the faster you are. this is compounded when that weight is taken off of the rotating mass of the engine. (lighter flywheel = faster revs)
the same will be true with the pullies. it wont be a ton but it will be something.

over all you just need to deside if you want to shell out the money to gain a slight gain, or if you want to save it and just get something with a bigger bang for the buck.

(forgive me if this makes noooo sense.. its way to late