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zooki.. Not too long ago, I was being absent minded and told the gas station attendant to fill up with $15 regular (87 octane and here in Oregon you cannot pump your own gas). I didn't realize what I said until he gave me the receipt. Guess what happened to the motor? Nothing. It was fine. I just didn't go WOT until it all burned away. I'm nearly certain that Toyota/Subaru have already accounted for the fact that people might do this occasionally. I'm sure the motor can be tuned to run on a lower octane of gas, but (as you prob already know) you will make less power. The motor is high compression. If we were talking about a 8.5:1 motor, different story of course. I have also researched differences in behavior in this regard with iron vs aluminum heads.
Good thought experiment. Just not sure that any tuners would be willing to put much effort into an 87 tune on 12.5:1.