Also, I understand the chopping method which is PWM and I'm trying to limit the current going through the motor.
Yes but the chopping method of regulation is based on the feedback from the actual current flowing. By simply applying a fixed PWM you have what is known as an open ended control loop, not a closed control loop. There are much better drivers to get the best out of your motor.
To get a motor to run faster you need to get the current into the windings as quick as possible. The only way to do that is to up the voltage. This means that more current will flow once it has reached the maximum, it is then that you must chop the voltage off. For your system you must find the optimum point of the open loop PWM where the current you want to use is reached. This requires you to look at the current waveform with a scope.
The other thing about stepping motors is they can go faster if you ramp up to the speed rather than try and achieve it from a standing start.