look into other catted downpipes that will mate right up to your catback. for instance, the helix and invidia downpipes are real nice pieces. these end in a 3 inch flat flange and should bolt perfectly to your hush. the hush is one of the best exhaust systems you can have.. i would hold onto that if i were other. otherwise, i'll take it from you lol. you want an sti catback?
i say look into other brand catted pipes to mate to your hush, otherwise, do what you were planning on doing.