I do own an ms3 and running nothing but 91 with the VERY occasional octane booster when I drive it REALLY hard I have never had even a hint of a knock...
If people want to waste their money on fancy gas that offers little protection and poorer fuel mileage that's fine, but its not going to help performance. thecar is tuned for 91 it has to be for California, the car will not recalibrate itself with a more aggressive tune for higher octane fuel if you put it in, if it did there would be no point in buying an after market tuner for it, because it would adjust for mods and fuel... Remember, this is a <$25,000 car, it's not gonna have NASA computers that adjust to every parameter, and mazda is not gonna waste their resources tuning different models for different states, especially because I'm sure California is a HUGE market for these and they would not want to exclude it because of available fuel...
93 won't hurt anything except fuel mileage, but fir this car in relativly stock form it's not gonna help