Compression check it. I feel like I've seen a bunch of people talking about this now, and I wish I had saved the trouble, spent the 3 hours to pull the plugs and checked the compression. I thought mine wasn't boosting, but when I'd check the numbers (after an ECU reset) I would still be pulling 16-17 psi, which was normal for the tune I had. It would feel like a misfire between 25-3000 rpm, but then it would smooth out and it just felt like I wasn't getting full boost anymore. Try taking off your oil filler cap when the car is warmed up, and look for it to be puffing white smoke. If you rev it, you'll see it blow out smoke like your breath on a cold morning. Well, you'll see that if you've got a piston/ringland problem. If you do see that, check your compression and see what cylinder(s) is(are) the offending one(s). My compression when the block was cold read 120, 119, 123, and 61. If you DON'T see puffs of white smoke at idle, nor smoke when you rev it with the filler neck open, move to the bottom of this post.
I hope that I'm wrong, and that it's something related to your ignition or front A/F (O2) sensor, and not a mechanical failure. Check the oil filler neck and move from there. I wasted almost a month waiting for Subaru to not figure it out, when I spent a few hours and found out the horrible news myself. I wish I would have just diagnosed it myself in the first place.
Other possible causes as I understand would be spark plugs, coil packs, or front A/F sensor.
PS: My car also would do a learning cycle and start pulling the boost so it would max out around 10 psi over the course of a couple hundred miles or so. Never once threw a CEL.