Jonathan

Have you bought the ballscrews yet? If not then get 10mm pitch (RM1610) for X/Y to get a higher feedrate and use HTD timing pulleys (although 1:1 ratio is about right for those motors and 10mm pitch screws adding the pulleys still helps as it reduces resonance). 5mm pitch screws (RM1605) will work but it wont go anywhere near as fast, or more importantly the acceleration will be less and possibly vibrate more as the screw is spinning twice as fast.
He is using an MDF box and i think the screws he has ordered are much more suitable, you need to stop pressing everyone with the need for speed? shake rattle and role will not bode well here.

Bruce your screws will be just fine for the application you have there.