Drive shaft?
 

I just bought a 1987 rx7 with a blown motor and I've decided to rebuild my na motor instead of doing a turbo swap because of cost. My only concern is a lack of power. So, to compensate, I'm going to reduce the weight of the car as much as possible. I've already purchased an aluminum hood from an 89 rx7. Did the 89 rx7's come with an aluminum driveshaft? Aside from lower rotational inertia, what are the other benefits of a lighter drive shaft?

No RX7 came with an aluminum driveshaft. Mazdatrix sells aluminum versions. There are no other benefits besides slightly decreasing the amount of energy needed to turn the shaft.

The inertia of the driveshaft is insignificant compared to the flywheel. The advantage of the Al driveshaft is a lighter car.

Essentially this is also true for a carbon fiber driveshaft, however the better vibration damping characteristics are a nice fringe benefit.