E85 can be a performance boost. Higher octane, you can turn up the boost. That said, you need more VOLUME of E85 than you do dyno oil. Your fuel consumption will increase. E85 should be (isn't always) considerably cheaper than dyno oil so this should balance out the added consumption.
E85 means you have to retune, limit your refueling options (tune for one or the other, not both), suffer lower gas mileage, and change some relatively minor fuel related parts. In return, you get the benefits of higher octane gas (more air, more power).
2015 STi: Still Stock
04 STi, 04 WRX: SOLD