O2 sims would be for eBay headers since they do not have cats on them.
And with most BMW cars you can get away with cats downstream because CA SULEV cars have 4 cats. 2 on the manifold and 2 downstream. The smog shop doesn't lift the car up and inspect that here in orange co. You just drive in on rollers.
Still seams like a lot of crap to deal with if I went that route. I'm still leaning on the stock m3 headers with a custom head flange. Cats and o2's would be in stick locations so no cel lights hopefully...