The extra 30 cents you spend on premium will save your engine, which could cost thousands of dollars to replace.
The ECU and engine was designed to run with 91 octane minimum. Could you run anything lower if you were in a pinch (say a couple of tanks of gas maybe)? Sure. But you would be a fool to floor it into the upper RPM band with low quality fuel...
No offense, but if the extra 30 cents of premium fuel is hurting you that badly, maybe you should consider selling the car and buying an econobox hybrid or a V6 model mustang? : shrug: