IMO, the biggest advantage to using E85 would be to run higher compression or bigger boost. Since changing the mechanical compression ratio is a bigger job than just swapping out a carb, there isn't much practical point in doing so unless you are building an engine up from scratch. If you engine is boosted, then you can just dial in more pressure with different pulleys or waste gate springs.
Since you will burn more E85 than gasoline to make the same amount of power, if your doing it for a mileage benefit, then you have to figure out the price differential where you break even. In my flex fuel Suburban, E85 needs to be $.65 less per gallon than gasoline for me to realize any financial benefits form using it. Sometimes that difference is there, some times it is not.
There is also the whole availability thing as well. Some places it is everywhere, some places it is not. There are only three stations in my city where I can get E85, so the hassle of making a special trip to fuel up is also a factor.
