You might try using the search function... this was a topic a few weeks ago that went on for awhile.
You are going to get different opinions on the benefits/advantages of using an aluminum d/s over the stocker...
Basically, whenever you remove weight from a rotating assembly, it will rev quicker since you loose some of the parasitic drag associated with heavier components... the end result is a few extra hp at the wheels, in short.
Also, I've read where so-and-so had a driveline vibration... swapped in a new aluminum shaft, and the vibe went away. Never experience this phenomenon first hand, but it sounds good.
Hopefully someone will be able to shed more light on the subject