Thanks for the fantastic and detailed explanation and description! Makes 100% sense. I guess if you're getting more precious/expensive material inside, it obviously will cost more. The CARB requirment is just one of the quirks of owning a car in California I guess!This is second hand info, so do not take it as the gospel truth. However, when I sold the guts of my precat and also my physically broken stock main cat to a local salvage yard, I got to talk to the yard's "cat man". He mentioned to me that the difference between aftermarket cats, OEM 49 states cats and OEM and aftermarket CARB legal cats is the amount of rare metals in them. The more of the catalytic metals there are in the unit, the better job it does of doing its job. Also, as the unit ages and looses some of its effectiveness, the units with more expensive rare metals in them will still do a legal job of cleaning up the exhaust gasses. The cheaper units with less rare metals stop meeting standards sooner.
Aftermarket cats have to meet a much lower standard of performance and longevity than 49 states OEM cats. All CARB legal cats (OEM or aftermarket replacement) need to meet an even higher standard of longevity and effectiveness so they have the most rare metals in them.
So, the cases and pipes are all pretty much the same. The higher up the chain they are the more rare metals are inside them in order to meet higher performance and longevity goals. That is why a salvage yard needs a "cat man" who can judge the type of unit pretty much on sight. A lower grade unit will have less rare metals in it and be worth less money to the salvage yard than a CARB unit would have, yet they all pretty much look that same from the outside. They do not want to pay you too much so they want to know what type of Cat you are offering them.
So, to your question -- they are all physically about the same but they do not all have the same expected service life or level of expected clean air coming out of them. The better ones look pretty much the same as the cheaper ones but they have much higher amounts of expensive rare metal inside them to do a better job of catalyzing/cleaning the exhaust and to do that better job for a long long time. A cheap one would pass a California smog check for a while, but would start flunking much sooner than a unit that was higher up the totem pole. To try to keep folks honest, the CARB folks require units they approve to be marked with that approval. They do not want sub-standard units that will start polluting prematurely.
Again, that is second hand, but that is my understanding.