The problem is that as you go up in altitude (DA or PA) the number of suitable places to do autorotations decreases.
Certification only requires autos to be demonstrated at 7,000' DA.
So no-one would spend a lot of time figuring this out.
There is an issue which has been little explored, which deals with blade stall in the flare/touchdown part of autorotations at high DA - several accidents that were attributed to pilot error were later determined to be stall of the main rotor, and there hasn't been much guidance published on this problem (note to self- more stuff for next edition of the book...)
Basically, you need to make sure you don't touch down with very low rotor RPM or flare too hard (when practicing, of course- if it's the real thing - who cares what happens if you can walk away?) when doing autorotations at high DA.
At high DA, your TAS is going to be higher than your IAS, so you'll have slightly more energy available in the flare.