I guarantee that you will get no better mileage with high grade , for the simple fact that highgrade gas doesn't contain as much energy as lower octane gasoline. The reason is because the octane boosters in highgrade gas (which is what makes it a highgrade gas) don't have anywhere near the energy capacity of low octane gasoline. People eternally confuse higher power levels with high octane, when in fact, the higher octane gas is more difficult to ignite and burns more slowly than lower grades - this is what allows high compression engines to run without harmful detonation. Ethanol, for example is now used to increase octane and ethanol gives significantly lower fuel mileage than gasoline.
For example : flex fuel models that can burn both E85 and regular gas (85% ethanol - it can't be higher because it would be difficult to start the engine on pure ethanol) have their EPA econmy ratings published at
www.fueleconomy.gov - a typical difference would be a vehicle
getting 21/28 MPG on gas, gets 15/20 when running E85.