If they are breaking gear boxes like that, it's the clutch, every time. I realize those guys are limited by the junk clutches those cars come with, but they don't make that much power. It's junk clutches that kill parts.
A good clutch will break parts just as fast, maybe faster, if the operator doesn't know how to run it. Something has to slip somewhere or it will break and/or bend stuff.
If the results don't match the theory, change the theory.
