I think the plastic Tamiya uses for the gears is not right. I have managed to modify the gear diff case to take a Yokomo SD crown wheel and seems to have no issues. The Yokomo gears are a sort of tough nylon different from that of Tamiya's. The reinforced gears are just a bad idea in my opinion. They are too rough and have too much friction. The plastic ones are too hard. They should probably use the same material they use on the bevel gears of their DF03 line of cars, I reckon. Those fell more like the material in the Yokomo gears.
To modify the diff case takes a lot of work and a good lathe, so not for the average modeller, but it can be done. The driving pinion had to be modified a bit as well to change the protrusion (plunge), but that was easy. My SD has run for a long time with these gears and there was never a problem (best sign is how silent, smooth and free the drivetrain is).
Too bad Tamiya seems unwilling to address these issues, the EVO6 could have been a good car. They might have just provided the best opportunity for their competition to come up with a good car in this market segment and Yokomo has the inside lane.