Not too sure about symptoms, but I remember reading somewhere if you suspect the cat is clogged up then try removing the 02 sensor and start the engine. If you feel exhaust pressure coming out of the empty O2 sensor hole then it is likely your cat is clogged.
Sure- no doubt about it, the O2 sensor is getting replaced regardless, Eric. Plus, I'm going to check out the voltage values at the new sensor to make certain I'm within range. The way this problem developed, and the way the performance went downhill so fast, I suspect the O2 sensor allowed things to go either rich or lean. From the discoloration, I'd have to guess lean.
The last thing I want to do is to do this job twice! Thanks.