I am going to say your CATS are fine. You say it is well maintained. Have you ever changed the O2 sensors? Have you ever changed the air filter(I hope so at that mileage), spark plugs??? All of these things affect "cat efficiency". You will be bombarded with these kinds of questions. I suggest you get your maintenance records out and start back tracking. If it was me I would attack the O2 sensors first. All of them if they haven't been done in the last 50K miles. Please keep this thread updated so others can benefit.

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Dave S.
Northeast
2003 4.6is
2005 E53 4.4
2003 E39 Msport(a REAL ONE!)
1959 Fiat 600