[Piwoslaw]

Originally Posted by roflwaffle

Offhand I'm guessing the effectiveness of the cat if it's a bit larger depends on whether or not the vehicle in question has a post cat sensor to monitor operation and the appropriate code to run richer and light off the cat faster if it detects excessive HC emissions at whatever time during warmup.

Ah, didn't think about that. If this is the case, then the richer burn will keep the catalyst within operating parameters, but at the expense of greater fuel consuption, which what using a larger cat was supposed to reduce in the first place. I'm willing to bet that in this case stock is the best way to go, unless you're ready to do some difficult and expensive A-B-A-B testing (which we would love to see the results of).