You will actually get a little better. higher octane burns slower... thats why its safer. Spontanious combustion of low octane fuel causes colision of exhaust gasses triggering sudden heat rise... basically taking a jack hammer to your motor when you use 87.

It really depends how you drive, the more your on the boost, the more it will kill your MPG. You drive cruising at like 1500rpm and you will hardly eat any

 

92 actually would give you better then running regular... it is beter gas.