you weren't far wrong though BT, if you lower the rpm you lower the friction between the piston and sleeve, bit like rubbing 2 sticks together to make fire lol the faster you rub the hotter it gets. I think what nitro dude meant is if you gear the car to hi it will put excess load on the clutch assembly and over heat and in turn raise the temperature of the engine.
I wouldn't bother with different gear ratios, just use the one that suits your track. just shim it as daniz24 said and run it a bit richer on the top end. also make sure the body is cut to give good air circulation for the engine to keep cool.