If I were to run E85 it would not be for MPG, but for power. When tuned for it, E85 engines can produce much more power than the equivalent e0 octane rating. Check out funny car racing, they run pure alcohol. There is one guy with an EB Flex putting out 600 HP at the wheels on E85.
Again of course you overlook the part that this is a Hybrid engine, where it does not function the same as a regular gas engine does, so BTU really wont make much difference, the end results would be negligible. I see your point in a regular gas car, 3-4% in a regular car is quite substantial, but in the hybrid, you wont even notice it. I also do understand Octane, the higher the octane the slower the burn rate, so running it in a high compression engine wont cause detonation like 87 would, however since cars can now adjust timing based on type of fuel, running higher octane fuels can result in better fuel economy, Case in point the F150 I had, Took it in because it seemed to be sucking gas, getting only 16 MPG when it should have been closer to 18, and I was told by Ford to run Premium if I want to see that kind of mileage. I was skeptical of course but did it any way, and they were right I was up to 19 MPG running on Premium instead of 87.
But this is in a Turbo charged V6, not an Atkinson 4. My Flex always ran on 89 Octane, and now so does my MKT, and both see 22 MPG or better(unless pulling a trailer, then its 12), I did run 87 octane in it a few times when gas spiked, and I did see a slight drop off, however, I also ran the same 89 in the Fusion since the place I got gas at, 89 was their lowest grade for the same price as all the others sold 87 at. Since that place closed I now run 87 in the Fusion and I see no difference in MPG.
Now for my lawn equipment and my gas powered planes, I would run non ethanol if I could get it, the seals and lines will last much longer. I had a line trimmer that I stored with fuel in it, and when I went o use it, the fuel poured right out of the tank, through a hole where the fuel line USED to be, the ethanol ate the fuel lines. Both lines disintegrated. Right now I use ethanol saver 2 cycle oil. Hoping it keeps the seals and lines from rotting out.