I tried a few tanks of 91 octane premium in my Mustang last summer. I have a Hypertech power programmer that can re-program my PCM for best performance with 87 octane regular or 91 octane premium. I had it programmed with the 91 octane performance tune. I got lower MPG with premium and the premium performance tune than I get with regular and the regular performance tune. There was a noticeable difference in performance, but the MPG loss "inverse squared" the price difference.
I'll feed my pony the less expensive "oats". Unless I'm racing..........