Lars,
OnSTEP is low cost solution because it uses cheap h/w platform (Arduino), and firmware (actually I should use Arduino term "sketch"
) is quite capable and still evolving (I am using 3y old version and it is quite OK for what I do). I have Pololu stepper drivers inside EQ6, and I am driving it from external Ardiono MEGA board via cable (so I can also use other controllers - Bartels, SoundStepper, my own controller....)
As long as you can reprogram your controller (via ASCOM) to suit your belt transmission ratio, you should be OK regardless of 400 or 200 s/rev.
I wouldn't know about G11 - but I know that stronger and heavier the mount is, the better (in principle) it is. The size of the worm gear is also important (bigger = more precise).