Someone told me a long time ago that rotating mass has more weight than static mass. So it may be trivial to shave 1 gram on the actual weight of the car, but if it's rotating, it's an exponential gain to shave weight on those parts.
Make sense? So if this is the case, any lightweight parts that are rotating would help with stock class. Hence, lightweight pinion gears, slipper plates, outdrives, and top shafts. I would think that all those together would make a difference.