To the OP.
I think that nobody will know much about cat back exhausts for non aspirated imprezas, to be honest. Not really the right demographic.
I guess that Norm Butler would fabricate something for you, but you might not like the price.
Just about any exhaust shop can sell you a Milo tin that will amplify your exhaust note.

Genie, Carline, Lukey and places like that will have something to suit.