I thought higher octane and compression gave you more POWER not more Efficiency?
From what I understand Diesel's don't get better FE because they use higher compression They get better FE because Diesel simply contains more energy per unit than gasoline.
Here is a list of Flex Fuel Vehicles ie vehicles designed to use E85 and every one of the cars that might lean toward fuel efficiency ie the small 4cyl engines see AT LEAST a 30% gain in FE from E85 to Gasoline
http://www.fueleconomy.gov/feg/byfueltype.htm
some of the lager V6 vehicles see a little less than 30% gains but I think thats mostly because they are larger heavier vehicles and can "use" the extra power (ie they get crappy FE from the get go)
For example in my club wagon towing a trailer or boat has ZERO effect on my FE because I have "surplus" power as it is and its a drag over power situation limiting my FE.