I have a 2012 Impreza 5-door base model, I've already built my own axle back for it. Anybody have a similar age factory WRX midpipe laying around that they don't want taking up space in their garage? My guess is that the WRX piping will be larger and better flowing than what I have. I found this to be true with my VW Rabbit when I swapped to a GTi factory catback.
It's hard to find just a midpipe but you should be able to find a car back relatively easily and cheaply.