A lighter driveshaft and input shafts would at best induce more torque steer, or introduce it in a car/on a track that previously had none. Comparing the weight of the motor armature and all the other longitudinally spinning parts, you should want to go slightly heavier on the drive shaft if anything. Why else would TC3 team drivers be using aluminum drive shafts, other than the carbon shafts breaking?
Personally I'd recommend sticking to the options that Hara/Thad use...which is to say, none
That said, I did put the RC Screwz stainless steel screw kit on my car, mainly just for a little bit of bling. I figure that should be it for me as far as options go, other than a motor heatsink.