New cat, passed smog with almost no measurable emissions
The shop told me that Porsche doesn't make an OEM cat for these cars anymore, and the after-market replacements are fine but they won't last 25 years like an original. They said they have been doing more of these replacements lately as the original cats are starting to wear out after all this time.
I emailed Steve Wong to discuss the chip I have. He recommended switching to one with a 91 octane map, and offered to throw in a chip programmed with the stock map for free, since I don't have one. So I went ahead and did that, and I'll sell the 93 chip on the classifieds here. He said the 93 chip vs. 91 would have no impact on emissions, but if the rest of the system (i.e. the cat, etc.) was old, a stock chip would give a larger margin for passing smog. I haven't switched them yet, though, and I passed fine with the 93 chip. In terms of using 91 gas with a 93 chip, Steve said that the risk is that there would be detonation/knocking under heavy load or hot driving conditions. So it's not like every minute I drive the car with the 93 chip I am doing damage to the engine, but for the long term it's better to switch.