Well this is no shock for us B8/B8.5 S4 owners, the P0420 (Catalyst System Efficiency) is a known issue because of the placement of the cats to the engine. The cats warm up fast for emissions, but get too hots when you are Stage 2 or higher... even on some stock cars. I'm been battling this error since going Stage 1, so it's nothing new. It seems to throw a CEL when I don't allow my engine to warming up before driving it. Per my research it seems like it's always the driver side cat that seems to have issues first, and my S4 is doing just that.
At this point I have two options, the first is to try Magnaflow's drop-in replacement cats, which would be $900+ for the pair... not bad, but still expensive. The problem is that they are untested, meaning in the Audi community, we don't know if they are just going to fail in another 30-50K miles like the stock ones.
The second option is to gut my stock cats and go cat-less. Obviously this is going to make the car stink, but any flow issues will be gone. Also, the state Washington is no long going to be doing emission testing starting Jan 2020 so the bi-yearly fear of passing will be no more. I can add aftermarket cats further down the system later on if I go this route.
I know you thoughts are why not go high flow cats like JHM setup and get the best of both worlds. The cost is the major factor here, as it would not be a plug and play fit as my existing magnaflow x-pipe was install in a way where the down pipes is now welded onto the rest of the exhaust system. Meaning, I would need to take it to a shop to install the JHM kit, and move the xpipe further down the system. As well the JHM seems to add a lot of rasp to the sound of the exhaust that I'm not a fan of. I think if I could find used kit this might be the route to go, or another custom cat setup. The cost will just be another factor. We will see...