gone postal

It's not going to make much of a difference at all, if any. The knock sensors will retard timing if needed to keep the engine running properly, you might lose a hp or two.

E-85 is actually in theory much higher octane than unleaded gas, the main issue in designing the fuel system to run E-85 is parts that are impervious to the fuel. Alcohol is corrosive as hell and high levels will trash regular fuel system components rapidly.