Update on my syncing issues:
I tested extensively today with an empty Live project - the Hapax syncs absolutely perfectly. Left it running for hours and it was perfectly in time (using audio syncing from a Mac to the Hapax).
I recorded throughout… it was literally hitting the grid in the same place from beginning to end.
Very impressed, and is now pretty much conclusive that my syncing issues are caused by buffer over-runs caused by high CPU usage!
Long live the Hapax!