I just had the code P0430 (Catalyst system efficiency below threshold, Bank 2) appear on my 2014 E350, it has just over 64,000 miles.
It's garage kept, I'm the second owner- purchased 2 years ago with 41,000 miles on the odometer, private party, after the original owner passed away. He took excellent care of the car as well.

I'd been getting into my groove doing regular maintenance (swapped tires, replaced brakes on all four corners) so like a fool, I threw parts at the problem first. I now have four brand new O2 sensors (upstream and downstream, replaced in pairs) and six brand new spark plugs (which I wanted changed regardless). Problem's not fixed.

Bank 1 is reading a consistent 160% on the scan tool every time I check, which is well within the upper and lower bounds (100 and 655). I've never seen it deviate more than 1-2% from that value.

Bank 2 is all over the place sometimes reading as low as 58% and sometimes as high as 96%
(Upper and lower bounds 80 to 655)

I'm not seeing any decrease in performance. It doesn't feel sluggish, it's peppy and as smooth as ever. After changing the plugs and downstream O2's, I noticed that the exhaust was visible, and smelled rich of fuel. After changing the upstreams, I've never seen this happen again even after checking obsessively. I'm wondering if it was a bad upstream sensor, and it ran rich and coated the matrix inside driver's side cat with gunk enough for it to not work efficiently, but not enough to obstruct the flow. The only thing that has me confused is that I wouldn't think that the reading from the sensor would be so inconsistent. If the catalyst matrix is coated with gunk, I'd just get a consistently poor reading. Varying by almost 40%, sometimes above the threshold, I don't know.
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In the spirit of transparency, I've used one bottle of cataclean already, according to the instructions on the bottle. I've used the Lucas Oil octane boosters before, but I chose that brand specifically to avoid this sort of issue, and I'm pretty convinced that if it was related to the boosters then the issue wouldn't be isolated to Bank 2. Mostly I'd like to know if the inconsistency is significant.
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Your ECU is flagging a lazy cat despite all new exhaust sensor collection plus ignition plugs.

I would try an italian tune up to help clean your cats. Upstream/downstream sensors are new already.

After that I would check what your new lambda sensors are doing to fuel delivery at idle:
Read idle LTFT to compare both engine banks.