You'd need to gain around 33% more economy to make up the BTU difference with E85 vs gasoline (though I'm sure it's cheaper per unit volume). I've personally found gearing to be much lower hanging fruit, and lean burn to be more for the last few percent. I increased the numerical ratio in my car's top gear by approximately 40% and found that to be worth something like 25-30% improved economy.
Which of course doesn't mean I'm not all running lean, or that I would discourage your experiments.
