Its a complicated explination, but its not just based on the octane rating, there's chemicals with octane ratings over 110, but dont do worth a crap in an engine.
Even if you didnt LOSE any hp....just the difference in how MUCH you burn isnt worth the swap. Say 1 tank of gasoline takes you 400 miles, 1 tank of E-85 wont take you nearly that far. MAYBE 325 to 340.
What I know of E-85 was told to me by the leading Octane Research guy, he trains more than HALF the US on octane testing and crap like that.
Go read a couple things here.
http://bayblab.blogspot.com/2008/05/octane-rating.html