My mileage SUCKS. I barely get into boost and im still only gettin 200 miles per tank. What kind of gas are you using getting 300 plus miles? For some reason the dealership told me I could use regular in my car, but I have been using 89. I guess I should try 91 or 93 but gas=too much money!!
I use pretty much whatever has the lowest price, but my best milage & performance was with Shell. The lower the octane that you use, the ECU will detune the car to that level, so 91 octane & higher, is the way to go. My 24-26 mpg is highway, @ 75-85 steady, but booting it when merging into traffic.