The accuracy gains between 8 and 16 microsteps will be marginal if any. The only time a stepper will produce full torque is when on a full step. In between those full steps, you're relying on two magnetic fields balancing the rotor over the magnetic detent. Stiction, friction, and cutting forces will all have an effect on the exact position the rotor ends up balancing.
Motor torque is directly related to current, not voltage. The reason voltage comes into the equation, is because as motor speed increases, so does back EMF. The higher the available voltage, the faster the motor can spin before the back EMF becomes too high for the supply voltage to push enough current through the motor windings. Once you reach that point, the current flowing through the windings falls off resulting in reduced torque.


Quote Originally Posted by ManicMetal View Post

Step, Direction and Enable opto isolation is provided at the stepper drivers. Polarity doesn't matter.
Maybe not electrically, but it does for the step/direction timing.


I use USB on all my machines, but I'm well aware certain installations can be plagued with problems.
The big problem is most USB motion controllers will cause Mach 3 to lockup if there is any kind of glitch/communication failure with the USB, at which point the only solution is a reboot, as there is no graceful timeout or failure.