I picked up this FWD Impreza coupe for $650 a couple of weeks ago, and I've only just started modifying it. The idea is it was already so ruined that I couldn't possibly make it worse, so ideally I'll eventually have a pretty aggressively streamlined car.
I went through a couple of tanks in stock form and averaged a little under 34MPG, which is already a pretty decent improvement over the rated 29MPG highway. I don't plan on doing ABA testing or anything that gets good data; the idea is just to make modifications that should improve economy and not worry too much about optimization.
Anyway, this thread is mostly just to document the process for myself, although maybe someone else with an Impreza might find some of it useful.