Found a little bit of time to wire up the motor on X axis (side to side one on the gantry) to see something moving.
I very quickly seamed to find the limit of these motors+drivers+screws at between 4000 and 5000mm/min (800-1000RPM at the motor). I was hoping for a little more than this. Acceleration is down at 20mm/sec2.
Do these sound like reasonable numbers when compared to other similar machines? Im a little more used to deal with lightweight 3D printers which are happy fly around at 150mm/sec+ all day. Guess a heavy CNC is a totally different animal though.

Spec is; 3.1Nm Nema 23 wired in parallel, 3.08A peak driver current, 68Vac, 1600 steps per rev (1/8th micro stepping), 1:1 belt drive, 1605 screw.

I know 1610 would be much more appropriate, but 1605 was sooooo much cheaper!
Motors are rated to 4.2A, so I could push the peak current upto 3.77A. Would heat dissipation become an issue running at 90% the rated current?
A quick google seams to suggest around 1000RPM is the limit before torque starts to drop of significantly, so im starting to think this is the genuine limit of the motors.

Its bloody loud too! Was expecting digital divers to be nice and quiet! I know increasing microstepping would help. What microstep settings do other people use?