I am only dividing the miles traveled by amount of gas that goes in. I don't know if that is the most scientific method, but it sure is ballparking it.You mentioned using a 2.0L impreza computer. That would work without other problems ONLY if:
1) It was a 2.0 4-EAT setup
2) The computers are pinned EXACTLY the same -- some brands you can do this , some brands you cannot. Do not know about Subaru.
3) The gearing is the same
4) It's also MAP
With that said, you aren't going to find a NA 2.0L impreza with 4.44 gears. So you'll have all sorts of speed sensor codes. So the impreza computer would be a no-go. You can get away with this on vehicles that use the tailshaft to set the speed, but something that runs off of the ABS sensors will have problems
The gas mileage you were posting isn't really terrible for a 2002 outback. Subaru's AWD system is not the most fuel efficient and the gearing causes it to scream on the highway.
I did put my Fumoto valve on today.
The most "screaming on the highway" car I had yet was my 4EAT Corolla. That car really needed its 5th gear.. My understanding is they got it in 97 or 98 and later, in the body style I like slightly less, in the Prizm/Corolla before whatever they did with it in 2002.
So. Is this another vote for the "leave it as it is"? I'd rather be coming out of this the right way, but we are having great discussion so, what would be what you would do. ?