if it will bolt on to stock, there are several downpipes such as cobb or turboXS that will also work with it.
high-flow vs catless
as for high-flow cats:
made his own turbo-back exhaust
The reason why I am sending you to his page is that he did Dyno testing with a high flow Cat and with no cats at all, and with the high flow he only lost 1 peak horsepower, but also a small amount of acceleration.
The Up pipe is insurance for modding... if you mod at all and don't replace the up pipe, your higher boost levels can send pieces of the cat through your turbo, thus killing it.
Without replacing the uppipe, the downpipe will not add all it can (because you wil not be able to safely turn up boost), but I believe without the uppipe, the downpipe will add a good bit of power even untuned.
as for the mid-pipe, there are 3 cats in the WRX system, most "Downpipes" replace the last two and some replace only the middle cat. So they might be called downpipe/midpipe combos. You dont see much about mid pipes because most people buy a turboback or a Downpipe that includes a midpipe