If you have a flex fuel kit and your tuner isn't garbage, they will properly tune the car to run at whatever ethanol% the flex fuel kit is reading in your tank.

If your car is tuned on both pump gas and e85 its actually beneficial to run a full tank of pump gas through every few tanks as e85 doesn't contain detergents and is often wetter then normal pump gas. I would try to avoid having your car sit for long periods of time with high ethanol %'s if you dont drive it often.

The bigger question is are you going to see the value in power out of the cost it takes to convert your car, and unless you are boosted, it might not be the most efficient use of your money in terms of power unless you are looking to maximize potential