Pulley Power?

How much of a gain do you really notice when you put the light weight pulley on?

taken from 3sx:

Unorthodox Racing lightened underdrive pulleys can provide gains of 5 to 12 HP and 4 to 8 lbs.-ft. on normally aspirated engines and 8 to 16 HP and 8 to 18 lbs.-ft. On forced induction engines.

I know that the underdrive will affect the electrical system, but how bad is it?

If you get the unorthodox which is an underdrive, you will have issues at idle with a sub. For this reason, if I were to ever do it, I'd get the 3sx lightweight pulley. It is lighter but has the same diameter as the stock, so doesn't underdrive the system. Check it out on 3sxperformance.com