I replaced my alternator and water pump pulleys with the Greddy Pulley kit.  This modification is a more controversial one, as  it may not apply to weekend warriors.  There are several purposes to the kit:

• Airpump Removal - This may not be a big deal but it also cleans up the engine bay

• Under drive the alternator and water pump - Both good and bad as no substantial statistics are available. 

The positive reasons for this upgrade are: cleaner engine bay, less load on engine when accelerating (slightly noticeable), possible reduction of cavitation from water pump (Basically if the water pump is slapping air bubbles around instead of pumping coolant)

The negative reasons for this upgrade are: Alternator running at lower RPMs (not a substantial difference but may affect large sound systems), cost versus actual performance increase is minimal

I chose to spice up my engine bay and install the pulleys.  I really wanted to remove my air pump as I had disconnected it a while back.  I was able to see my first turbo after removing the air pump.  I am keeping all of the old parts in case of emissions.  My engine bay looks sharper after the upgrade.  I also did notice a lower fuel consumption at idle.  It was extremely small but the PowerFC caught it.  ( I used to idle at approx 3.8% inj duty, now it is 3.6%)

I felt a smoother shift into first gear but this may be due to unknown factors. 

Pictures