What they all said...I have Trifecta's tune for E47 which is 1/2 E85 and half E10. My car runs stronger on the 50/50 than just premium only because it is tuned for it. Without the tune, you are not sending enough gas/ethanol/fuel to the engine making it run lean. Ethanol needs to flow about 30% more than gas to run properly. I would add premium fuel every chance I got and drive very easy until most of ethanol is gone.
On most factory FFVs, the efficiency of ethanol does not outweigh the cost meaning per mile costs of ethanol are higher than gas unless you really can get ethanol cheaper. In the midwest, ethanol is 15-20% less than gas but is about 25-30% less efficient so it is close but still loses. Here in the south, ethanol is 10% less than gas. I use 50% of it because I make more power. I was getting about 28mpgs with premium tune and now get 24.5-25 with half ethanol.
09 G8GT, Stryker, sport/prem
08 GXP, Mean, Auto
03 Excursion Limited, V10, 2WD and 37s
94 Bronco XLT, 5.8L, very rusty and 35s
07 GXP, Mean, manual-RIP 1Apr13 @14K miles