I'm pretty sure the cat in the ZZP header is a metal spun cat designed for high-flow applications. Putting an OEM cat in place would just restrict exhaust flow. Also I'm pretty sure one of them is part of the manifold, if you have one.
I have two of them, i'm not sure about an '08.
Just take a look, pop the hood and look behind the motor, If it's like mine with two cats you'll see one still in the engine bay, then look under under the car you'll see the second o2 sensor and then the second cat. then the flex pipe.
If you had just one cat you would see the first o2, cat, second 02 then flex pipe.