Ian - the grbl wiki seems to suggest that even the uno (328P based?) can get up to 30KHz, which is pretty good going. Quick back-of-the-envelope sums suggest that with a 5mm pitch ballscrew and 800 microsteps, a 16KHz pulse rate would give 6m/min. That's a pretty good performance per pound out of the little 328P chip and even in terms of absolute performance is close to what I get from my LinuxCNC machine. Be interested to hear your findings.