I switched my talon over to E85 and ran it that way for years. I could run any mix of e85 and pump gas I wanted. I had tuning software that connected my car to a laptop and all I had to do was adjust the global fuel scale to keep the fuel trims set properly. I am new to modifying mustangs so I don't know how many different parameters each different tuning setup can control.
E85 where I live is actually e70. The air to fuel ratio for ethanol is dramatically different from gasoline. You will need to increase your fuel injector size dramatically. For example a 43 lb injector on pump gas is roughly equivalent to a 65 lb injector running e85. You also need to have a fuel pump that can flow a much higher volume of fuel. I don't know the limits of a stock mustang fuel pump.
I liked e85 a lot more than race fuel. It is dramatically cheaper than race fuel, makes more power, and it is highly knock resistant. Ethanol has a lot less energy per unit volume than gas, but since you are injecting a lot more of it you make more power.
If your fuel pump and injectors are e85 compatible there is no need to switch them out. On my talon I ran e85 on a walbro 255hp fuel that wasn't designed for e85 without any troubles (there is a long story behind the wal bro pumps). My injectors were also not designed for e85, but had no problems running it. Since a lot of factory vehicles have ran e85 in the last decade I wonder how many vehicle fuel systems could run it without a problem... I wonder if the s197 fuel system will not be harmed by e85... I honestly don't know.
The only problem with e85 is that out West it is not as readily available for a daily driven car as it once was. I absolutely love e85 and not once did I regret switching over to it. It was a lot easier for me to tune my car on e85.