Serial diagnostics aside for a moment, it is quite easy to evaluate catalyst performance with a simple tailpipe emissions test (surprised you didn't mention it Pete )

I preffer to look at tailpipe readings from cold start & the monitor it as the engine warms up whilst idling.

A fully functional catalyst will tidy up the exhaust gases quite quickly, whereas an inefficient catalyst will take a bit of prompting (increased RPM to achieve light off) before the readings become good.

I am not sure about the politics & legislation behind EOBD Catalyst monitoring, but I am convinced that it is too strict in some instances.

I have several regular cars that have had P0420 logged for 3 - 4 years & they still pass the MOT year after year with ease.

Maybe a PCM software update, addresses & alters the "threshold" at which a catalyst is deemed to be "inefficient"?

It's not the first time that I heard that Toyota have an update available to deal with an overzealous monitoring system.

I am sure this topic is deeper than most of us realise.