Yes, belt pitch makes a big difference. As I said, I used XL, which is 5-pitch. Again, with a stepper, you'll be hard-pressed to get better than *maybe* 25 IPM. A large stepper is NOT going to go fast (maybe 1000RPM at the very best), and torque falls off rapidly with speed, so you'll likely become torque-limited long before you reach that max speed. With a 3:1 reduction, 750 RPM would give you 25IPM. If you have to go to 2:1, you'll likely be looking at 15-20 IPM max.
Regards,
Ray L.