Screw pitch on X and Y is 5 turns per inch. Steppers are 2000 pulses per rev, so 5 x 2000 = 10,000 pulses per inch.
As I mentioned, I can get the X axis to at least 200ipm with no lost steps if it's running by itself. The problem comes in when X and Y are running simultaneously, as happens for circles.
Going from a 4A to a 15A power supply does not make any difference.