Bit of a PBI there " engines designed can adjust for all grades of gas, based on its sensors pioneered by the knock sensor". This is true only if the advance maps can accomodate the different fuel (higher octane needs more advance, lower octane less).

The problem is that as a lower octane needs more advance, if it goes into detonation, the knock sensor will pull back the timing when what you really need is a higher manifold vacuum (MAP).

This is where the flex fuel capability of my Jeep is nice because it can handle anything from 85 octane to 115. I doubt that my Crossie's maps are that extenive. The tuners should know.

Personally I usually alternate 89 and 93 filling at a 1/2 tank.

btw the secret to high compression and 87 PON is variable valve timing. This lets the engine avoid detonation by dialing back the maximum cyl pressure without affecting the timing. Engines designed in this century have it.

ps if you monitor the injector pulse width with an OBD scanner you can develop a feel for the AFR. A good scanner can interpolate IPW, MAP, TPS, and a load value to generate the AFR. Or can just look at the O2 transitions.