how will ethanol affect my fuel mileage ?

Fst73_360 said
IDK about "significantly" but you do get less. Friend with STI 22 pounds of boost gets around 250 miles a tank

Well, 250 miles off a WRX tank (over 16 gallons, 16.9 I believe), is bad. That's under 15 MPGs from a car that should get high 20s.

That is significant.