[KenWittlief]

One of the limiting factors of electric motor rpm is the centrifugal force on the wires, armature, commutator...

With no load you can keep increasing the voltage, and the motor will spin faster, until you reach the point where the motor rips itself apart.

I think that is the real issue you will face.

Heat dissapation is the second. You can push the HP rating of the motor as long as you back off on the duty cycle, to give it time to cool between application of full power. This would more of less be a matter of trail and error. The failure mode for a motor that overheats is usually the wires melting/unsoldering themselves from the commutator, which is not easy to do since most motors are soldered with silver solder, and the wire is hooked or twisted to give it extra mechanical strength.

right in the same range of abuse, the varnish on the wires will start to cook off from the heat, making a wonderfull pungent scent that all FIRST teams have experienced at some point. If the varnish burns through and wire shorts to wire, then the motor turns into a space heater, and you get those wonderfull clouds of white smoke spewing like Mt Saint Helens.