I'm no E-85 expert. But, I've read that the % of ethanol changes, when the weather changes from hot to cold. I'm assuming that means that the % of the mix will be different, in different parts of the country, at different times of the year.
E-85 was not made to give hot rodders a higher octane fuel for their high compression engines. It was supposedly made for modern, flex-fuel vehicles. I assume that the original purpose of ethanol, as a fuel, was so that we'd use less crude oil. It got real popular when gas went to $4 per gal, because it was cheaper. A different gas/ethanol mixture is no problem for a flex-fuel vehicle, since everything is computer controlled.
But, some hot rodders discovered that E-85 has high octane & runs cooler. And since most racing gas is around $10 or more per gal, E-85 is much cheaper. So, it made a lot of sense to try it in high CR engines. A lot of racers have used it & like it. My engine guy built a 4.5" Pontiac stroker, for a modified dirt track car I owned. On E-85 it made 755hp @ 5900rpm. It ran strong on the track & never came close to overheating, even after many consecutive laps.
E-85 carbs are readily available. The main concern with using E-85 is the ethanol content variation. The E-85 guys use ethanol testers, so they'll know the ethanol content of the E-85 they buy.
For seasonal variations, I assume that jet changes may be about all that's needed. All the race fuel companies offer racing grade E85 that's always a true 85%, and has more octane. But, it's only a little cheaper than race gas, therefore wouldn't be of great benefit to street guys. Some street, street/strip, & race only guys are using E-85 & love it. So, it's an option for those who wanna use it. For others, they can just stick their head in the sand and say it's no good. The choice is yours.
