I diagnosed based on the boards here. Had the P0420, but also had residue around my exhaust exit. Had overheating issues associated with a loss of power when overheating. Couple that with the fact that it accelerated like a slug, I found someone say on here that those point to a bad cat.
So then update on this: replaces the cat today. Stupid place I ordered from sent me the wrong part (I was missing the port for the upstream o2 sensor and the rear flare on the pipe wasn’t big enough to fit around the old exhaust pipe.
Solution: salvage the old pipe from the manifold to just after the upstream sensor/before the old cat. Cut the other end of the old pipe the appropriate length. Crimp this end of the old pipe so it fits in the flare and weld the joint. Repeated at other end (didn’t need to crimp that end, just welded it in place). Attached the mounting bracket (the only thing that lined up right), put in new sensors.
Result: I have power on acceleration again, I have no overheating issues, the welds are holding solidly on place (I’m so much better with an arc welder instead of a mig welder, but didn’t have access to an arc), and most importantly… no codes.