Your car was designed with the ability to run 10% ethanol. All cars built from about the 80s onward are designed to easily run 10% ethanol. If you can, I'd suggest you run 10% ethanol, as it promotes a better burn and keep the fuel system cleaner.
As for mileage changes on E10, there are none. I tested this in my 97 GT and I've read articles published by reputable sources that state the same. In fact, most cars on the road today can run up to E30 without a problem and some cars even GAIN mileage on mixtures up to 30%. While it doesn't make sense from an energy balance perspective, the ethanol promotes a better and more complete burn and of course allows for more timing and a more agressive tune because of the octane.
There was a station near me selling E20 and E30 and I tried it out in the 97GT. At the time, I drove about 400 miles a week, so I knew what my mileage was and it was very consistent. On regular 87, E10 or E20, I would consistently get 20-21mpg. When I was running E30, it dropped to 19-20mpg. I have since moved, but I would still run E20 or E30 if I could. In fact, some states are considering making E20 mandatory and the current testing shows that modern cars (80s and up) will have no problem with this.
Before I started running E20 and E30, the 97GT would ping BADLY. If it was warm outside and I did a hard pull in 3rd or 4th gear, it would ping so badly I had to let out of the gas. While running the E20 and E30, the pinging went away, of course because of the higher octane. Now, I'm running streight 87 and it still doesn't ping! The ethanol got rid of some carbon build up in the cylinder or gunk in the fuel injector so it runs right now with not a hint of detonation! The car has always pinged on a hard pull since my dad got it about 5 years ago.
As for running E85, I would NOT advise it without a tune. When you mix 1/4 tank of gasoline with 3/4 tank of E85, you get around 50-60% ethanol. I am surprised the computer can compensate that much without throwing a CEL, and there is no way it will compensate for that much ethanol, but there is no way it will run correctly on 85% ethanol. In fact, what is advertised as E85 can have up to 90% ethanol.