Are you asking if it would affect gas mileage when running 87 octane gasoline compared to a standard engine, or are you talking about comparing the economy of a flex fuel engine when running 87 octane gasoline vs E85?
Let me answer both just to cover all the bases. :) E85 is less fuel efficient than 87 unleaded, by a minimum of 10% in engines highly optimized for it, and more like 30% for average engines. This is because E85 has quite a bit less fuel energy than unleaded gasoline.
A flex fuel engine running 87 unleaded should deliver roughly the same economy as a gas-only engine.