A larger Battery will increase your ROF, burning out the motor is relative,
If you are doing long Ratta Tat Tats, with an 8,4V you are sending a less number of BB´s down Range, by increasing the Battery size you will send more BB´s Down Range in a shorter amount of time, the motor works faster.
It is an improvement in Performance, but with every Improvement made on an AS Gun, the wear and tear is also increased, regardless if it is an improvement of FPS or ROF.
You can safely run your gun on an 9,6, like L_W stated, the bushings will give out before the Motor does.