I believe that stronger, hardened steel toothed gears in the 393 motors would be great to put in an upgrade kit. In my experience with driving my robot with high speed motors I have the tendency to strip out the internal gear, and this is very frustrating, and is also getting to be expensive.
It always seems to be the same gear that strips out too. And in the repair kit there is only one of those gears, so replacing the gear leaves you with a bunch of gears that 1 you don’t use, and 2 leaving you to buy another kit once the repair gear strips too.
Make the internal gears of both the high speed 393 motors (1.6:1) and the regular speed (1:1) out of a stronger material, or sell the internal gears individually, instead of in a kit. It seems to me though like eliminating the problem at the source with the material of the gear being stronger would be the best.
Hope you all take my suggestion under consideration.