I have the 5" aluminum driveshaft out of Virgil's SS...total weight is 17 pounds.
I'm not sure what the stock ones weigh but it's considerably more than the aluminum one even though the diameter is a lot less.
I'll have my stock one when I get my truck back in a few days and will post up the weight.
Not sure how much of a difference it will actually make, but obviously lighter rotational mass is going to give a little bit better performance than heavier rotational mass.
Every little bit helps...