I run a G540 so in my understanding, I am 10X microstepping. I agree with your comment about resolution however I am constantly corrected in regards to the G540 and its morphing. I had thought about changing to a direct drive and I have seen it done on this machine quite nicely. But after all is said and done, I still only get a 5"x5" envelope. I might just settle for optimizing this machine with its present running gear and build a G0704 or RF45. Everything is always up in the air in these regards. The machine is beautiful and so well thought out and executed. Hard (read impossible) to find in a hobby class. This machine was built for the educational field in my estimation and they had and endless stream of money. The retail on my little mill was over 15,000.00 in the late 80's, thanks to ebay my cost was very good! It originally came with auto oiling on all axis', limit switches, auto backlash measurements on each axis upon initializing, probes and quick change tooling based on an ER16 spindle nose.[

The biggest limitation on you speed with that machine is caused by the gearing. The ration is so high that you are spinning the motors somewhere near their max RPM If you want highewr rappid moves you could change teh gearing to a lower ratio and gain speed for rapid moves. There would be some loss of torque, but at the ratio you are using that probably not be a problem.

Another question - Are you Microstepping? At the number of stepps you are currently running you could turn off microstepping, change teh number of stepps per unit to match the new setting and use that to  run at a higher speed.

at 100,000 steps+ per unit you have a lot of resolution that you probably can't use.