E85: E70, E74, E80...changing pump mixture a problem for tuning?

[scheides]

So through the seasons at the pump E85 is offered in various flavors. In the summer it's full 85% ethanol, and in the winter it's 70%. This then means that the target AFR will vary between 9.7:1 to 10.5x (?) as the various seasons come and go. I'm considering switching my evo over to E85, and I know it's possible to re-tune the stock ECU to run efficiently on E85 w/ ECUFlash or AEM (which the car will eventually get).

The issue I see is this change in mixture through the seasons. I don't think it's a small enough change to 'not really matter' in a car like an Evo, where I'll get it tuned for high boost and strong timing. Will a re-tune be needed for each new season's mixture? Or is it really close enough that it won't matter?

Call me a tree hugger, but the simple fact is that E85 is produced locally and was made for a turbocharged application, and I'm considering giving it a go.