An easy check for a bad cat is take the precat o2 sensor out and tie it off to the side and go for a drive. Yah it will make some extra noise but if it runs fine it will tell you that it is a bad cat. Removing the sensor releases backpressure that a bad cat can create. Try it with both side if you have a cat on each side.
Have you actually done this?
The pre cat "upstream" O2 sensors are what primarily affect A/F ratios. I don't even think it would run without them properly in place. Also, I think exposing the O2 sensors to the fresh oxygen while running can ruin them (although maybe that's only when they're hot...not sure...).