OBX headers require major modification to work.

Skip them and get the SSAC's

and yes, it'll still work on the 04+ cars.

If you dont have testing, and aren't worried about the fine, don't bother buying a cat, just use the pipe they come with.

If you're concerned about it, get a 3" I/O high flow cat.

theres only a small sound difference between cat and no cat so I wouldn't really worry about it.