So hub motor ratings are literally just that: a number that gets stamped on them.
Technically for a 1000w motor it means that if you put 1000w through the motor it will not increase the temperature of the motor beyond X amount. That's it.
Generally hub motors are rated for 150% of the voltage they are designed for (which is a factor of the windings of copper). Other than that it's just heat and eddy current field management.