by a "chopper controller" do you mean a PWM.
but why are you using a chopper on a stepper.
The reason you would use a higher voltage is because your stepper motors will be more responsive (higher peak RPM, etc.). Again there is a limit to how high a voltage you can use but without the datasheet for the motor it's hard to say; ~40V is typical.
Are you saying I can put 40 volts into a stepper motor rated at 12 volts if I use a chopper drive?
Take note: the motor you have linked is a five-wire, unipolar stepper motor. Your Pololu driver is for bipolar stepper motors. The two are not compatible. A bipolar driver will only work with 4, 6, or 8 wire stepper motors.
The only voltage limit is what your chopping regulator can cope with.