I've got my 912 uls to a bit over 11500 briefly. -6 OAT!
As per the other posters, it ran perfectly but those egt's were so awfully cold. Maybe around the mid 600's.
I was probably wasting 4 or more litres per hour......at least.
Whereas a nice WOT run at sea level, on a "high oxygen" day can get me easily over 800 degrees C egt....so sweet!
As I normally fly at "altitude", I have considered buying a HACMAN but I've not pulled the trigger yet.
http://gsatech.store/hacman-mixture-control-kit/