I am running the subachad pipes, they are expensive but they make things so much easier. If you go that route make sure you get the downpipe extended by 4", the pipes were designed for the impreza and makes contact with the legacy's subframe if you don't.
IF you really wanted a cat in your downpipe a muffler shop could easily weld one in for you. You will need to have a small portion of the exhaust system made up to connect the downpipe to the mid-pipe and could always put the cat there.