One other thing to consider - perhaps the cat is actually clogged? Check for "normal" exhaust pressure at the tailpipe. Try partially or even completely sealing the exhaust pipe with a piece of metal or stiff cardboard. If doing so has little to no negative effect, the cat may be clogged.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280