Update:

Spoke with the folks at Petersen. Like I suspected, they tested the "high compression" O-320s with 89 octane, no detonation, no pinging, zip, nada, zilch. They brought the science to the FAA and got shot down. "91 or else" said the FAA, as always more interested in CYA than the truth. So they had to budge if they wanted the STCs approved. From the horse's mouth.

Completely unrelated but now I feel much more comfortable about my healthy skepticism for government numbers.