I guess the principle reason that slews don't get there in the cheaper mounts is down to backlash. If the original slew error is an overshoot, then the correction may not be effective. Thinking aloud, is there a way to use syncs say like we do backlash control on focuser movements? So - if centering is enabled, the initial slew is split into two parts. A deliberate overshoot and partial correction - which takes up the backlash. The centering then corrects the remaining error without encountering backlash?
Just a thought...