As his car is a 323is, he will have a check engine light because his car is OBD2. He has 2 O2 sensors, and the one after the cats are there to make sure the cats are doing their job. If the cats are plugged or broken the sensor will detect this and throw a code and check engine light. Only way to fix this is to get an O2 sensor simulator, but your engine performance still won't be optimum. I actually have seen test pipes online for sale online, but they're almost as expensive as a new set of cats. Bottom line is if your cats are broken it is highly recommended that you just replace them.
Cars are much more compact if you don't take them apart.
There is a theory which states that if anyone ever figures out how and why the universe came to exist it would instantly dissapear, and a less rational, more complicated one would replace it.
There is another theory which states that this has already happend...