BTU's are a great way to measure the ability of a given fuel to boil water - they're less accurate as a way to measure the amount of power available to drive a piston.
While an engine optimized for gasoline will suffer poorer mileage when fueled with ethanol, an engine optimized for ethanol (generally a higher compression ratio and advanced spark) suffers lass of a mileage drop and has more power available.
It might take a little time to optomize all the vehicles out there.