The Forester XT or Legacy GT would make excellent sleepers. One of my friends commented on the new Outbacks having ram-air. (He's a domestic guy, it seems ram-air is the only mod they do to their V8s.) I quickly corrected him that it was actually a hood scoop for the TMIC on the 250hp engine.
As far as toning down the STi, I'd just do the trunk lid swap, hood swap, and debadge the exterior. I wouldn't go into trying to make it impossible to figure out it's an STi, just enough so while you're passing someone they don't notice.