Page 94, Owners Manual... (GS-R model)
"If you are unable to find premium unleaded, you may substitute an unleaded regular gasoline. The engine will compensate for the lower octane, but you may notice a slight decrease in power as a result."
Use your butt-dyno, let me know if you notice a difference. MPG's are virtually the same, i've been driving on 87 octane for years now. Got my oil analyzed, engine is perfect. Not to mention, cars only need the higher octane fuel some 5% of the time when you drive, and that's usually only when you are towing or have lots of weight in the car and are going up a hill.