your gantry is pretty light compared to what i built. mine is around 300 lbs and i have the central drive with a Nema 34 stepper currently driving it and i don't see any issues with what i have but i an running at a 4 to 1 ratio with a timing belt rather than a direct drive. the stepper motor is running pretty slow and it's in a better spot on the torque speed curve. i was able to get up to 400 ipm at the extreme side before lost steps come into play. direct drive on a 5 mm pitch screw i would expect to see fall around 100 to 200 IMP you may get closer to 200 since the gantry is light but the stepper is not going to do well at a higher RPM. and the higher the voltage you can feed it the better.
your ball screws are a little shorter than what i have on the gantry cross axis of what i built and it does ok up to about 400 IMP then the the screw starts to whip. there are ways around screw whip so it's not the end of the world just a little more complex mechanical end of things as a result. but since yours are shorter than 1500 mm they will do better.
one thing that might help is some cross bracing on the frame under the table. if you can get some decent acceleration it works on the frame more than you think at first. one other thing is the drive beam under the table should probably be more substantial. with the center drive you need to be pretty solid so you don't get into problems. at your machine size the central drive is not to bad. on a 8' axis it's a completely different animal. the central drives need the entire gantry to be pretty solid.
anything other than Mach 3 is a step in the right direction. it was been a corner stone for a long time but is getting closer and closer to it's end of days. Mach 4 seems over priced. acorn centroid looks like a good plug and play solution but i have not tried it myself. seems like these days there are a lot more choices as far as control software is concerned.