If you have never done it take off the throttle body and clean it. The '04+ Drive By Wire cars are known for having issues with gunked up throttle bodies and it causes all sorts of issues...such as rough idle, hard starting, sticky accelerator, etc. That's free too. As for the cat if your dead set on that being the issue...you can get a universal magnaflow converter off of ZZPerformance or off of EBay for under a hundred bucks shipped to your front door.