ask a fellow subaru tuner. Tuning for E85 wouldn't be a problem, that's pretty common amongst subaru performance lovers. You could tune it extremely conservative in terms of timing advance BUT with the nature of E85 itself, you're gunna go through more fuel than if you were to tune it for 87 or 91 specifically.
Typically E85 cars get worse mileage, though the fuel is cheaper. So I don't know why you'd go through the trouble of doing so.
what BAC said...