Hi Neil
Having used a relative focuser (Skywatcher motor on crayford focuser) on my Newt and an absolute stepper driven rack and pinion on a refractor, I'd be concerned if SGP could do this reliably for you.
Not sure if you've seen SGP in operation, but effectively:
1. Do a rough manual focus
2. Run auto-focus
3. SGP moves focus n/2 defined focuser steps away from focus point
4. Then iteratively moves focuser back through focus point 1 until n/2 x steps out the other side
5. Plots slope of calculated star HFRs either side of focus, then commands focuser to move back to the calculated central point
Bottom line - you need reliable movement, with negligible backlash.
My old Newt rig wouldn't have done this reliably, particularly if pulling a camera up against gravity at some angles......
Hope all that guff makes sense
Presumably you're already getting some sort of reliability from Backyard EOS though.