BB's and shims will probably solve your wheel torque issue. That or put some "blue" loctite on the axle threads, then tighten the nuts up as tight as possible, with the wheels spinning freely, and let it set up overnight..
Who told you you cant change spur gears?? Redcat might not have one to fit, but other manufacturers like HPI probably have some that'll fit it.. Also, you can change pinion gears to accomodate the hotter motors..(Go up in pinion teeth to gain top speed/less take off torque) Not sure about the heat issues.. But if the ESC says it can hangle 12turn motors, it should be ok with them.. Just need a heatsink for the motor, and also make sure the drivetrain is free from binds or excess friction and make sure the mesh isnt too tight when ya install the new motor.. Good luck!