I've found this on my brushed motors running with Robinson pinions. So I don't run a full mesh. I would think the Tamiya pinions (there's one included with the kit) are ok but I've never tried it because it is undergeared for what I'm running. I'm sure there are pinions out there that would solve your problem.
BTW, post this in the Tamiya area and you might get a better response. It will also be handy for other 501X owners in future.
