I wouldnt go any higher in that case, I rule of thumb i use is this... you want to hit top speed by about 1/3 of the way down the straight. there is no point gearing right up, only to hit top speed just before the end of the straight, and then you lose your momentum and cant really get it back through the infield...
if your motor is getting really hot (too hot to touch) the heat will damage your motor, get a heat sink and fan.
just my 2 cents worth...