David,
I've had 3 or 4 Zf gear sets and ring and pinions cryo treated over the last couple of years. Two of these sets ar currently running in Jim Click's original GT 40.
He has run multiple events ont the gears with no noticible wear normally seen after many races.
There is a gentlemen here in Phoenix who does this service. Contact me directly and I will get you his contact information.
Les Gray