I have the 2.2 liter and I'm not getting much better mileage about 20 - 22 City 28 - 30 highway.
Subarus run notoriously rich, even with the proECM it's still gona run rich, unless you can find the perfect fuel to air ratio your mileage isn't gonna get any better.
Plus the 91 octane is kind of wasted money in a N/A car, I actually get better mileage from 87 or 89.
Bry.