One can get direct spindle control w/ the SO3 controller: http://www.shapeoko.com/wiki/index.php/Spindle_Control#Speed_Control — Grbl supports this, and I’d be surprised if Carbide Create can’t be set to send through the command for spindle speed.
I’ve had quite good repeatability on my SO3 — I’ve re-run some jobs at just slightly lower depths and things seemed to line up perfectly, throughout the cut.
Tim Foreman on the SO3 forums is getting his machine upgraded a bit, and is getting quite good numbers.