The supercharger doesn't engange until WOT, so until then, technically the engine acts as a naturally aspirated engine (with the same compression ratio as the NA engine), with a supercharger to finely tune the amount of air entering the engine.

I've watched my boost gauge, not even surpase 10 vac when I've accelerated. My theory is that the only time it would be best to have premium gas is during WOT. Other than that, why not use gas that is simply easier to combust. Our engines have ECU's to deal with the variables of heat, air, and fuel. By using the 87 octane gas, I'm having the engine have to send in less gas per cylinder, which means less air. For the love of god, our engines were created in the 80's, just because you slap on a supercharger that only works during WOT, doesn't mean you need gas constatly prepared for WOT.