Hi dunnitagain,
I've been thinking about your spindle speed problem, I see five different solutions
1) Use Sensorlles Vector control if your VFD is capable.
2) Use a pulse generator, aka your encoder, to provide feedback for your VFD, if it is capable.
3) Use a F to V chip and opamps to close the loop electronically, fun if your electronically inclined.
4) Uprate your controller to give analogue output and use your controller to close the loop, costs of the upgrade?
5) Use Mach4s built in PMC, probably result in less control bandwidth than 2) or 3) but still probably tens of Hz,plenty enuf.
Craig.