What is the output power of your 36V PSU? How many motors and how long are they? Do you know their voltage and inductance?

I doubt that the steppers are the problem.
You have a 20 tooth pulley with 10 mm pitch? This means you are moving 200 mm with one motor rotation. At 800 mm/sec your motor has 240 RPM which isn't that high.

At high speeds acceleration is important. Unless you are cutting very large parts, you will hardly get to your max speed. Better the acceleration, quicker you'll finish the job with the same feedrate.