Put a back pressure guage in the o2 hole and watch it freewheeling at 4k, if its over 6-8 psi drop the exhaust right behind the cat and retest.
I know I have seen monolith beds shatter and restrict mufflers in the past and if the cat took a hit on the bed casing you might have a chunk in the muffler.
One simple test thats sure to raise a few eyebrows in your neighborhood, drop the exhaust behind the cat a go for a drive. (make it a short one)
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.