Possibly this is a silly thought but is it worth trying to getting the X and Y axis ball screws machined so that they can be used with a fixed BK bearing on both ends? The price difference of the bearings doesn't seem to be much and it would increase the critical speed nicely. The only problem I can see is that I'd have to be quite accurate with the positioning of bearings.

Any recommendations on stepper motors? I'm currently thinking the 4Nm steppers from CNC4YOU look quite nice.