I'll swap the thermostat
I did notice on the Torque Pro app that the coolant temp would drop to around 175*F when ideling and around the same time the CEL came on for the P0420.
I'll swap the current thermostat out for a 195*F one later this week when I get it back from the shop getting the lift installed and see what it does with the code. Hopefully that fixes the issue.
It's plausable, the engine not warming up enough could an overly rich condition which can contribute to inefficiency in the Cat.... I'm still leaning towards your cat not getting hot enough to work as the cause of your code.