its not amps that give you speed with steppers but voltage and inductance.

No stepper will be using full current at 150 IPM. Most will be drawing less then a third of there rated current.

For a given voltage system lets say 50V the only thing that will determine top speed is the inductance of the steppers.

For 300 IPM I think you need to spin a stepper at 1500 RPM with direct drive.

This is a chart of a 6.1A 640oz stepper.

http://www.automationtechnologiesinc...80-45-8ATC.pdf

As you can see it makes vary little power at 1500 RPM.

You could gear up you stepper or go to a faster screw but its hard to get steppers to work well at 1500 RPM.

The 570oz nema 23 stepper is really good in this way.