Octane is around 105. It is true that a vehicle will lose some gas mileage with E-85 unless it has an engine specifically designed for it. With the high octane, one could raise the compression ratio of the engine significantly, and this would not only increase power quite a lot, but it would gain back most, if not all of the fuel mileage lost.

High compression is what we call a "no lose" modification on an engine IF you have the fuel to run it. It impressively increases engine efficiency across the rpm band, whereas many other common engine mods cause the engine to lose low rpm torque in order to gain horsepower, and usually cause worse fuel mileage as well.

AV8TOR