If there's no spark knock on 91 oct, than 93 or higher won't be of any benefit to the engine or performance, but it sure will improve the profit margins of the folks selling you race gas or oct boosters.
I live at 7,000 foot altitude, and our premium is also 91 oct because our thin air makes for lower effective C/R, and the only time I ever notice some light knock in my 99 CSVT is on very hot summer days, which I attribute to some air temperature related octane sensitivity. In the winter, when the air is cold, I can run 87 or even 85 oct without any perceptible knock.
When I was driving cars with carburetors and distributors back in the 70's and 80's, we used to set the dizzy to factory timing with a timing light, then we'd advance the dizzy a few degrees at a time until we got some light knock pulling up a hill in high gear, and that told us we'd advanced the timing as far as we should go. A little light knock never hurt anything.
