driveshaft

think of it as reducing weight. the lighter your car is the quicker it will be, even if its only marginal. however with an aluminum driveshaft you will not only lose weight, but will also be reducing rotating mass, which essentially frees up power, ie. more power from the engine makes it to the wheels. dont expect to feel a sotp improvement though.

so if the price is right, its worth it.
 
Aluminum driveshafts tend to be more precisely balanced. If you have gears the driveshaft tends to be rotating at a higher rpm at any given time. Therefore, gears and aluminum driveshaft kinda go together.