As far as i know most aftermarket companies build their exhausts around the factory exhaust dimensions so it should be fairly universal.
I don't know what that exhaust is, but judging by the lack of any muffler I'm sure your neighbors hate you. The only reputable company I know of that makes angst cannons for the Subaru is nameless. Most other companies do the giant grapefruit launcher exhausts when volume is more important than sounding good.
I'd opt to keep the resonated mid pipe if putting in a downpipe. That thing is going to drone so bad you'll be a permanent migraine.