I've filled the HAH with 1/3 ethanol by volume now (6.7gal E85).

I expect to see a 10% hit in mpg. But I'm more curious to see how VCM reacts- will the car sense more power and go into eco mode sooner or more often? Or will there be some confusion and eco mode will be twitchier?

I don't think there will be much if any perceptible change- 1/3 ethanol isn't all that much of a change- probably effectively about 94 octane. I'm not willing to go with straight E85 at this point.

I picked a poor time of year to do this, though- shifting to winter blend, rapid temp changes, all make for several variables at play that can cloud my results.