[notfarnow]

can't wait to hear the experts chime in, I was wondering about this too

My understanding is that the cat doesn't affect *performance* because there is nothing measuring its operation & output. It cleans up the emissions, but doesn't give any feedback into the ECU to affect driveability or power.

The exception is if it's plugged and causing excessive backpressure, but that is a mechanical issue, not a sensor/computer one.

Mind you, this is my understanding based on 10+ yr old cars that I've owned. Probably the newer, more complex cars measure post-cat emissions