P0430 - "CAT Efficiency Below Threshold, Bank 2"

I've got that error, and had it for a while already. I've had it on my other vehicles before as well, and the fix has always been to replace the downstream o2 sensors. Well, I did that a few weeks ago on my Taurus and changed both downstream o2 sensors (replaced with Motorcraft units) even though only one was bad. Why, you ask? Because my train of thought is to "replace it in pairs". Since getting to the rear o2 sensor's pig-tail harness was quite an adventure which required removal of the upper intake manifold, I decided to replace all 6 spark plugs (replaced with Motorcraft platinums) as well because removing the intake manifold is part of the same job to replace the spark plugs. I wrap everything back up, clear the code, and go on for a 65 mile drive, ~50mi on highway, and ~15mi on surface streets. The code stays off and I'm a happy camper. This was about 3 weeks ago.

Fast forward to just this past Tuesday and I get another CEL. I get out my ODB-II scanner, and I'm surprised to get the P0430 code again.

Reading online what some people have experienced when getting this code, symptoms vary from nothing noticeable aside from the CEL being on, loss of fuel efficiency, to misfires, and very sluggish performance. My experiences seem to be just the CEL being on. Some people have said they've needed to replace their CAT while other's have just needed to replace the o2 sensors. And here's where my confusion is. Some people replaced the pre-CAT o2 sensors and others have replaced the downstream o2 sensors (like I did). Some people replaced the CAT all together.

What troubleshooting do I need to do to figure out what it is that I need to fix? Some have suggested to swap the o2 sensors around. Some have said to replace the upstream sensors. Those are good ideas which I could try, but what else is there for me to try?