I put a Pro Drive back box that was off a wrx onto my XT and it bolted up no problems. The only thing you need to worry about is the 3 hangers.
The 2 that hold the box under the bumper need to be a fair bit longer than the stock ones, otherwise you'll wind up melting your bumper. The 3rd hanger further down towards the flange wont line up at all with the car.
I wound up getting a one of these from halfords - http://www.ebay.co.uk/itm/Halfords-HEX404-Universal-Exhaust-Strap-Rubber-Black-11-5-292mm-Long-Spare/321680633007?_trksid=p2141725.c100338.m3726&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20150313114020%26meid%3Dbfffad41324b4a148a2e878e615b0606%26pid%3D100338%26rk%3D3%26rkt%3D17%26sd%3D321680632982
Which i cut in half, drilled out the holes and used them as my new rear hangers. Then for the 3rd hanger i just used some metal cable ties between the cars original hanger and the one on the backbox, this was just to brace it so it doesn't sway left and right.
I've been swearing that I'll go back and tidy up the cable ties and make some proper hangers but it's been working fine so far and I'm lazy :P
Should only be an afternoon job as long you get a friend to help with positioning the new back box and make sure you have something to heat the bolts when removing the old back box because we spent over an hour on 1 bolt until we heated it for 30 seconds and it came straight off.
Also don't forget to get yourself a new gasket - http://www.ebay.co.uk/itm/2-5-Exhaust-Decat-Pipe-Flange-Gasket-Subaru-Impreza-WRX-STI-Legacy-Outback-/220720290256?hash=item3363f459d0
Jelly