I realize that as forum members and 4C owners, we all live in different parts of the world where we all have different rules and regulations to live with. Here, where I live, removing the catalytic converter is illegal. Sure, we could put back on every two years when we have a mandatory smog check. Unless the car is a dedicated track car and gets removed from the system, it's just not worth it.
A better path would be to install a high flow converter. From the new cat, a large bore single and straight pipe would be ideal. Magnaflow builds the cat-back, Centerline is the only distributor. Mine cost about $1000 USD, but I stayed with the stock cat. The kit includes new grills and the flex pipe. I haven't installed it yet, the few that have says it's got a nice sound and the car feels faster.
I think Madness sells a exhaust of the same type.