Given that GAMI had an awful lot of trouble making a TSIO-550 detonate on various fuels, and having to go to a CHT of something like 500F to do it, it doesn't at all suprise me that one could fly just fine in practice on 91UL - so long as it is "avgas" i.e. doesn't have the high altitude and the quality control issues.
On any realistic flight profile, where one climbs at Vx or Vy for the shortest possible time (due to CHT management) and then trims for a decent speed, and where the power is dropping off pretty fast post-takeoff due to the loss of MP, it cannot be an issue, IMHO. Legally... it's different.