There are very few things that are known for certain.
RiotSC gears are the best, period. Prometheus is second best, but Pheonix might be comparable. Systema gears are still considered good (their QC is good but the actual strength is crap). Everything else ( CA, Guarder, Element, XYT, TM, etc ) is a bitter knife fight of opinions and experience.
Tightbore barrels, hopup buckings, hopup nubs, and BBs are never going to objectively reviewed, and this is due to the fact there is no control group available. You would need an indoor range, a dozen people, and a ton of time to test every permutation of barrel/bucking/nub/BB to find the answer. You'd also need to consider barrel length, hopup setting, and FPS.
Pistons are a pain also, but might be easier to review. Just adjust for optimum AoE, and take pictures every couple thousand cycles until failure. Maybe run three tests per piston against 330/400/550 FPS springs.
Motors aren't really too mystifying. The tricky part is applying the data to an AEG gearbox, where the torque required changes throughout the cycle. <Taken from
http://www.fastelectrics.com/elecmotorbasics.htm > The chart below shows max torque at 0 rpm i.e. stalled; Max power is at 50% of peak rpm; Maximum efficiency at approximately 75% of peak rpm (90% for a good brushless motor) and additional curves for amps and current. An electric runs at peak efficiency where power is less than at the peak. When the load on an electric motor increases and rpms drop torque increases.
