Wow! Too many variables. :) For firmware speed caps I would post a separate question to the firmware forum of your choice. Make it simple.
For a leadscrew system I'm not sure you gain much going from 16X to 128X microstepping. It wouldn't really make any difference if the smallest real step is say 0.1mm and 16X stepping does it in 4 steps while 128X does it in 32. Also if you are looking for speed, the faster you go the less those microsteps are going to matter.
Just to add to the confusion, power supply voltage will affect top speed as well. Higher drive voltages increase the speed at which back EMF equals drive voltage.
For a leadscrew system I'm not sure you gain much going from 16X to 128X microstepping. It wouldn't really make any difference if the smallest real step is say 0.1mm and 16X stepping does it in 4 steps while 128X does it in 32. Also if you are looking for speed, the faster you go the less those microsteps are going to matter.
Just to add to the confusion, power supply voltage will affect top speed as well. Higher drive voltages increase the speed at which back EMF equals drive voltage.