I can see where Darren is coming from in questioning Kev about his comment "the ECU may have Adapted to old cat" as I dont think that in this case it has any bearing on the fault.
However I can see the theoretical argument that Kevin is putting forward. A partial blockage in a catalyst due to an insecure monolith may restrict the throughput of exhaust gasses to such an extent that the resultant pressure could have an adverse effect of the inducted charge. i.e. If the exhaust gasses cant get out of a running engine then conversely they are going to struggle to get in.
Take the example taken from the childish prank of stuffing a potato into the exhaust of a car to annoy the miserable old guy when it wont start. Then compare that to a normally running motor & somewhere in the middle you could have a car that appeared to run reasonably well, especially with the PCM’s ability to adapt or work around anomalies in its environment. Now if you remove the said restriction, the PCM would be faced with some different inputs from crucial sensors, namely the MAP or MAF & possibly the O2 sensor.
We have seen or heard examples of what can happen when a PCM struggles to deal with it’s new environment after some modification or clearing of learned values & it is this that I believe that Kevin is driving at.
I agree with Darren in as much that, on a system without a downstream O2 sensor, then a catalyst failure due to efficiency & not structural failure can have no effect on PCM adaptations.