Yes - The alt pulley is lighter than stock, but like Aldive said, it's not a big issue. It being lighter is not going to do much, but in theory, you're thinking correctly. It's just so small you won't feel anything and probably won't see a difference on a dyno. In other words, I just replaced the ALT pulley with the "Over-drive" version about a month back, and didn't feel any difference what so ever (In performance that is) However, this did fix my charging issues I ran into. A bunch of people are running just the underdrive crank pulley with no problems at all, but it depends on what you're running. With my e-fan, and that's about all I'm running that's non stock, I needed the overdrive ALT pulley.
The underdrive water pump pulley is supposed to slow your water pump down just a little more. Honestly, I'm not sure how much you get out of this because it's always been on my truck since the crank pulley, and I've never had a problem with it.
All and all, it sounds like you have a good idea what this does for you.
Also, the underdrive crank pulley will give you one of the best "Seat of the pants; I've got more power" feeling you will have as far as bolt-ons.
If I could do it over again, I wouldn't change a thing. I like running all the pulleys.
