You are going to lose about 30% gas mileage if not more because you are going to be hooked with more power and MPG wont be a concern. Don't think, DO IT! By your 811 Turbo Cobra sig, I would say you would be ok with the fuel system. Bet on 30%ish more fuel needed.
I would get it as low as humanly possible, because you would be diluting the mixture away from E85. Around here, I have a station that consistently puts out E90 so more could be left in. There are a few ethanol calculators on the internet that you will be able to see what your ethanol content will be with how much gas you have left. It would be hard to know exact gallons though.
The only problem I have with e85 is that when I start my car on a cold start I has a rougher idle like missfiring but after it runs smooth as silk ...I believe most e85 cars do this..correct me if iam wrong and if so who tunes for this not to happen.
You will need a retune.So if I go with E85, do I need a re-tune? Or is everything just alot safer?
Does the % change alot?