ASG and most of the other reputable workshops can tune an oem
ECU in the GD WRX/STI to suit e85.
It is highly recommended to go to an after market
ECU to run e85 as it is a lot easier to tune and you also get the fun goodies - antilag, rotational idle, launch control etc.
To run e85 you will need an e85 compatible fuel pump and injectors. Also worth doing your fuel lines to suit as ethanol eats through plastic over time.
A ethanol content analyser is also recommended although not necessary. I have been running e85 for 4 months now with no problems and have only upgraded the fuel pump and injectors.
If you are doing it to an 01 wrx it would be worthwhile to have the supporting mods done - cooler, turbo, exhaust, intake, headers and remove your catted up-pipe... Otherwise you are wasting your time and money.
Hope that shed some light
using the Perth-WRX mobile app