By pins I don't mean the motors, the motors themselves will take 6s no problem, I mean the subsequent power they will put out will most likely cause the gearboxes to fail. This type of gearbox has plates that have pins on them, on which the gears spin. The second set of pins are under major stress from the torque of a motor, and in our experience (and endless examples of the same over the years) the monstrous torque produced by an overvolted motor causes them to loosen and literally fall out of their position. 8/10 times this will lock up the gearbox, quite possibly strip several gears, and if the motor continues to receive power for long after being unable to move it will burn out.

If more speed is what you're after, I'd recommend doing the conversion of 36:1 to 24:1. :)