When I was at JC we would do CAT efficiency tests with two five-gas analyzers to learn basic diagnostic skills. My jeep at idle and 2500 (no load) put out low enough emissions to pass smog, before the CAT. After the cat the readings were even lower. Heck even on the dyno in 3rd gear WOT ~2800 RPM the jeep was only putting out 61 ppm HC and .05 CO don't remember NOx.

What I'm saying is an engine with a good tune on it should pass smog with a bad CAT. The cat is there to take those good emissions and eradicate them as much as possible. If you are running way out side spec the cat will burn off as much excess as possible. Unfortunately, this raises the temp in the CAT. Even if it doesn't burn it up quickly, the CAT will be outside its nominal operating temp and will burn out eventually.

You can always throw a CAT at it and pass smog if you are border line over. But if the condition is severe enough, you might burn out an expensive cat in 2 years.