wouldnt do it without a tune. you won't be able to compensate for fueling demands and AFR.
even if you get away with 15/20 % ethanol, the hassle of using two pumps won't be worth any measurable reward in performance and it will only hurt gas mileage if anything. sure it could help with some stock timing maintenance but without a tune I doubt that's even an issue with 91-93 octane on stock boost levels.
