If you have a Legacy Turbo mid-pipe it will mate right up. For that matter, your non-turbo midpipe will mate to the downpipe as well, but is only about 2" diameter. For long term plans it may be worth your while to spend a little extra money now, and do this:
Then when it comes time to upgrade your downpipe, any WRX aftermarket unit will bolt in. The greatest gains to be had with an exhaust are the parts closest to the turbo.
I know a guy here in CO, that runs something really similar to my design on his WRX and runs 12's at 5000ft. People told him there was no way he would hit those numbers without a full straight through 3" exhaust. But he got a really good down pipe, cut his stock WRX pipe and tapered the piping like I did and retains the stock muffler, just like I did on my Leggy Turbo. The moral of the story is don't believe everything you read on the interet...