None at all. The RM4's Gearbox and internals seem to be pretty beefed up compared to other brands.
If it ain't broke don't fix it
However I do know guys who have upgraded the motor on their RM4s though.
I never felt the need to do it. Then again I'm not all too fussed about having a high ROF either or having a really quick trigger response.
btw can you snap of photos of your rear wiring work? I maybe interested in doing it myself