Running an electric motor beyond its ratings
It is probably no issue to  run a motor beyond its voltage rating.
Electronically seen, you might exceed the break down voltage between two adjecent copper wires in the armature, but it is quite unlikely at this low voltage.
Mechanically seen, as voltage is proportional to rotational speed, a higher speed burdens the bearings more. Contineously running a motor at higher speed will not directly damage the bearings, but the bearings will wear out faster.  

Running a motor beyond its current rating will heat up the motor. You can temporarily run it beyond its current rating provided you don't exceed the thermal limits. If the motor becomes too hot, the isolation of the copper wires of the armature degrades or even melts, causing shorts between the windings, which on its turn increases the motor current causing a thermal runaway. The point when this happens depends on the cooling of the motor housing, ambient temperature, etc, so, not easily to predict.

You cannot do simple power calculations with motors: when the motor is running with no load, the current will be low, so the input / output power will be &lt; 500W. When stalling the motor, the motor voltage will be close to zero and the the current high, but still the power will be way lower than 500W.

Read this answer to see how the power and efficiency are related to the torque and only have a local maximum.

To make useful assumptions for a motor, you need its torque vs speed / current / power / efficiency graphs (as in the linked answer above) AND/OR its physical constants (speed constant, torque constant etc)