I went on a trip with the car last weekend, I had two partial tanks of gas (~10 gallons each). Just 87 octane, not premium, no E85. I think I detected some pre-detonation on acceleration onto the highway, hard to tell.
No CEL's on the last tank with ethanol, but I only got down to about 3/4 of a tank. My next tank will be 4 gallons of E85 and the rest 87 octane gas, just to prove one way or the other that I get a CEL or not over about 25% ethanol, which is what I found during the winter.
I'm now running about 13% ethanol in my tank, car running great. No issues at this time. Almost 159k miles on the odometer.