The stock cat is reportedly a pretty high flow design to start with, so I wouldn't replace it unless necessary. You can get a Catco high flow cat for about $90, or a Dynomax "super converter" for about $110 on the net. I believe Magnaflow (?) also has a performance oriented cat. I'm not sure, but 2" pipe may be OK for a Zetec. You could save money on pipes and just replace the res with something that flows better to get a little better high RPM breathing.
