Doesn't the OBX header require straight pipe to replace the cat section? could be wrong but i thought the OBX unit came with a custom straight pipe to replace the cat section. I believe DC has a header that mates to the stock cats, if you require them.
I do, but am looking to add a header to mine probably next year, to eliminate the ridiculous choke point at the collector, and net a couple more ponies, followed by a custom air intake and a lightweight flywheel when the clutch finally lets go.
The DC catback is a nice inexpensive stainless system, though mine IS rusting lightly, mainly at the hangers. It drones a lot and loudly between 3-4k RPM... unfortunately right at highway cruise speeds. If the noise is too much, The Silencer that Versaspeed sells does a really nice job of quieting it down, but you do lose a lot of the performance gain.
I also recently replaced my stock cat section (rust) with an Eastern High Flow "Direct Replacement" unit. Fitment wasn't as "direct" as they would lead you to believe, but with some muscle and swearing, will fit.