Tonight I finally put a full tank of E85 into my car, and the flex-fuel functionality continued to impress me.
I drove around for about 30 minutes while datalogging so I could make sure everything is behaving (before Wednesday when my car is booked in for an E85 tune at MRC). According to the datalogs, which included full-boost on a few occasions, it looks like MRC won't even have to change any fuel trims, they can just focus on ignition timing and adding a few more psi of boost.