If I was to run an electric motor rated at 24V 20A at 15V 10A and at 10V 15A what would the difference in performance be? Assuming everything is rated to safely handle more than that power.
You cannot "run" a motor at a certain amperage, because of the two fundamentals:
voltage determines speed
torque determines amperage
Okay, but why I can't select the torque then?
You can, but because both the motor and the part driven have a torque over speed characteristic, they automatically run at the crossing point of their characteristics, the working point. At that point, the torque of the drive and the countertorque of the part driven are balanced so the speed does not change any more.
That means the torque determines the speed. You can play with the voltage to ramp up the speed, but get another torque balance then, which most times also means the amperage goes up as well.
All motors have a characteristic curve that shows the motor's torque capability vs. the motor's operating speed. Driven loads have a similar curve that show the torque required to drive the load, the torque demand, at a given speed. The actual operating torque and speed is the intersection of those two curves.
To a great extent, reducing the voltage applied to a motor creates a new torque vs. speed curve for the motor with a lower speed at every given torque point. For some types of motors, there are other factors that have an effect on the curve. For a permanent-magnet DC (PMDC) motor with a commutator the other factors are relatively insignificant. A group of torque vs. speed curves for a PMDC motor is shown below. The curves correspond to various voltages increasing from left to right. For this type of motor, the speed increase is quite linearly proportional to torque.
The maximum torque is relatively constant regardless of speed, but the safe operating time at lower speeds is limited by the cooling air flow produced by the motor itself. The maximum speed is limited by the insulation voltage, the commutator construction and mechanical considerations.