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Too many variables. Anywhere from nothing to 150+rwhp. Depends on setup. If the blower is maxed out, and you have good gas/meth already, gains will be almost nothing besides safety. If you are on crap 91, and have more room to crank the blower up/etc, you can gain as much as you want before something else becomes a limiting factor.
E85 will just remove any octane limits, so you will gain whatever limits your car next, assuming it is octane now.
702rwhp on 91 @9psi
722rwhp on E57 @ 9psi (some 91 and some E85, I wasn't there to drain tank and tuner didn't feel like draining entire 91 to get it to just E85, flex fuel sensor read 57% supposedly)