How are you going to tune it with the Tactrix cable? I assume you're paying someone to do it? I would at least get a downpipe. The cat isn't really the issue, it's the connection to the back of the turbo. You want something with a bellmouth or divorced wastegate path (nearly all aftermarket downpipes have this). You don't really need a new cat back exhaust at this point. You should be able to research which downpipes will work. Just about any of them should be fine if you use the GrimmSpeed adapter.