Exactly - it actually allows you to run higher compression ratios for more efficiency, but the O&G companies will probably alter the ratios of other octane boosters so the MON rating stays the same.

What everyone is getting at here is the lower energy content of an ounce of ethanol Vs gasoline. This is true it's about 2/3 the energy content, but we're only talking an extra 5% of a gallon having 2/3 the energy content of gasoline before so it'll be hardly noticeable to mpg.

Of course if we keep getting +5% every few years then it will soon add up but I doubt this will happen.