You will notice some power gains from the Pulley upgrades. The Underdrive Pulley will get you the best gains because it free's up more power that is normally used to drive the accessories.
The main reasons to stick with the Standard Sized Light Weight Pulley are:
A) You are running a large aftermarket Stereo system that draws a lot of power.
B) If you are planning on going with a Supercharger later on (the underdrive will under power the supercharger if used).
Is there any concerns with harmonics and crankshaft bearing wear due to the lighter or under drive crankshaft pulley. I am concerned about causing premature wear on the engine for minimal gains.