I am not real familiar with the cats on 16v cars. The standard 2.3 cats have NOX precats right at the connection to the exhaust manifold and use the engine heat for the catalytic action.
Most aftermarket cats are about 1/3 the size and have all three components under the car. I have no experience solving NOX problems as we have no testing at all in Florida, but its concievable that the aftermarket cats do not perform well for NOX due to size and placement. The oxidation section of the cat is always under the car and was probably the part of your original cat system that was bad thus giving the appropriate oxidation values while dropping the reduction component.
Just a guess.
Bosch Master, ASE Master, L1
33 years MB technician