What do you need to run e85 in our s550 without losing performance?
Stainless steel fuel line?
Bigger fuel pump?
Fuel filter change, probably stainless steel?
Just curious if I were to switch what would I need.
Just a tune. Modern vehicles already have the needed equipment to safely run E-85. E-10 and E-15 is becoming more and more mainstream and is actually required in some places. So the manufactures made everything E-85 safe. Since the mustang is not a flex fuel vehicle, it can't adjust for different fuels so you need a tune for E-85 and you can only run e-85 with that tune.
Actually if you get a good tune you will actually gain performance. E-85 has an average octane of 105 so the person writing the tune can crank up the timing to take advantage of the higher octane. And as long as your mechanically stock or close to it, the stock pump and stock injectors can handle the increased volume of fuel needed for E-85. Also unless you pull the heads and mill them down and or change out the pistons, your compression will not change.
If your want to play it safe, get a boost a pump or something similar. They basically send like 16v to the pump and overdrive it forcing the pump to pump out more than its rated for, just make sure your tuner knows you have one so he can compensate for it in the tune.