Help Me Diagnose P0430 Cat Issue…

So I’d been chasing a misfire issue for a while now and after running a couple of cans of SeaFoam Top End Cleaner through my intake manifold the misfire issue has disappeared. However, not long after running those 2 cans through, I’ve been popping code P0430, which is the post cat O2 sensor reading “Catalyst System Efficiency Below Threshold (Bank 2)”. I did some online searches and looks like if the exhaust is hotter after the cat, that the cat may be fine. Pre cat, the exhaust is around 230-240 while idling, and post cat, it’s right around 400. That seems to indicate the cat is OK, correct? The car seems to drive fine, but while idling a do hear some stuttering out of the exhaust. Could this be a rear O2 sensor? I thought the only thing they did was monitor emissions. The front 02 is what tells the engine how to run correctly, but I’m not getting any pre-cat codes. Should there be more than a 160 degree or so difference in the exhaust temp pre and post cat indicating I may still have a cat problem?
Thoughts and/or suggestions?