How is your fuel economy? Is the car running rich?

This link (pg 10) has some info supporting the idea not to automatically replace a cat for an efficiency code without doing some additional checks.

http://www.epa.state.il.us/air/vim/air-repair/air-repair-review-06.pdf

If there is a vac leak, marginal O2 sensor, dirty MAF sensor, etc. then as Klaus says, the mixture can be a little off. For these reasons or if there was a previous history of misfires, etc., the cat can become contaminated. If the original problem gets fixed the cat could possibly clean itself out after extended driving. I had this happen. After a year or so though the code is back. I'm now looking for other reasons not to replace the cat.

Some cheap aftermarket replacement cats aren't good enough at the outset to guarantee no efficiency codes will appear.

Good luck...