My understanding is that as the cats are in the cans on a Roadster, changing the pipes out will eliminate the cats. What you have is an empty cat box in the middle, sometimes called a pong box (or something), which I understand has some baffling in it to help balance the 2 pipes and provide back pressure. But no cat. However, Staintune make a freer flowing cross over pipe which is a 2 pipe version of the 3 pipe cat eliminator needed on pre-Roadster models.
And yes, Dave's right, you can't fit the 3 pipe system to a Roadster because of the ABS. Well I think that's right anyway.