I would get both cars on lifts and take a bunch of pictures. There may be issues besides bolting to the engine. I did that kind of a swap on a Volvo one time and discovered (the hard way) that the trans rear support was built into the chassis and that chassis for autos were different from chassis for sticks. I ended up having to cut a bunch of steel channel and weld up an adapter bracket.
With more modern manufacturing techniques it seems unlikely that the chassis on the WRXs are different depending on tranny, but pretty much anything can happen over four model years. Good luck.
Pitfalls include different wiring, possible depreciation from a swap, different drive and half shafts, keeping the trans computer (so the main ecu doesn't say "nope"), adding pedal and neutral safety switch.