so anyways, this might sound stupid to some, but please fill me in. so what is the difference between a cat-back and just direct bolt-on exhausts like the HKS ones? also, can you turn the HKS ones into cat-back? hope im making some sense. becasue i read on the column when they tested out the xerd products, it was said that the IS had a HKS Hiper cat-back. thanks guys.