The engine is not designed to run E-85, as it will eat the seals, and you'll void the warranty. Any particular reason why you want to run E-85? There's more power to be had, but less availability.
As for the power, 500 HP isn't much over stock, so you're probably fine with the stock drivetrain.
2016 Race Red GT Premium Fastback - Performance Package, Navigation, Enhanced Security Package, Ebony Premier Trim