Would you be running 100% E85 (which is 85% ethanol 15% gasoline) or a blend of E85 with petrol?
At higher concentrations of E85 you may need to change to a compatible fuel pump. You will certainly need bigger injectors and a remap as lean best power is achieved at 8.5:1 AFR whilst rich best torque is closer to 7:1. Cruise fuelling will need remapping as stoich is ~9.8:1.
As a consequence you will take a negative hit on mpg.
There's a huge thread on one of the impreza sites about the use of E85 which will answer all your questions, but I'll just have to go and locate it.