boy do i love this subject.....
It's all about a little thing called Parasitic Drag, which is when your engine's power is used to turn certain parts of your drivetrain in the process of getting that power and torque to the rear tires and down to the pavement.
Starting with the engine, lets say it makes 300hp. From the flywheel, it has to use some of it's power to turn everything inside the transmission (weather auto or standard) which then turns the DRIVE SHAFT which of course then turns the rear end which turns the axles and then to the tires. The part of all this that has to do with your aluminum drive shaft is that it weighs less, and when you're talking about parasitic drag, weight means everything because the less something weighs, the easier it can turn, which then "frees up" power, since it doesnt actually "create" power. In the end, your motor has to use less energy to turn the drive shaft which is a conciderably heavy piece of your drivetrain, and that gain in energy equals an increase in performance. Man i hope that made sense. Basically, take the stock shaft and do 100 bench presses with it, then do the same with the aluminum.....you get the picture.
Oh btw, i'm not sure of the validity of this part, but i was told by the guy i bought my shaft from (SVO) that he has run 1200hp+ through his and that they're basically indestructable, except for the U joints and input shaft, but you'll have those problems with any drive shaft. I've beat the piss out of mine but then again i'm not makin all that much power. I've seen 8 second Mustangs run the same model i have though.