If the highest available in your area is 92 octane then you probably won't notice much difference between 91 and 92, but you're still only talking a difference of $1.70/tank if it's empty.
With the different availability of grades of gasoline, they have to make the Porsches run on any octane fuel. Running and running well are two different things, though.
The biggest performance killer with the lower octane fuels is the knock sensor. Its' job is to help save the engine from being damaged by knocking in the cylinders. The lower octane fuel is more volatile so it explodes too early from compression and not from the spark causing the knock. The knock sensor feels knocking occurring and the O2 sensor notices left over fuel going down the cats. The computer starts changing fuel/air mix, timing, etc. to make the fuel combust when the spark occurs and to make sure all of the fuel in the chamber is burned. That kills your performance because the engine was designed to run best with the higher octane fuel.
A lot of people hate the knock sensor because of the retarding of performance it can cause, but I love it. Imagine all of the pressure put on the cylinder when it's on the way up and the gas explodes early putting all of that force down on the piston before it is supposed to. It's kind of like trying to throw your car in reverse while you are still moving forward.
The next time you're in GA head over to Road Atlanta. They have gas stations around there with 104 octane fuels.
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