I appreciate the input. The car is a U.S. version, does not have an EGR and has no signs that it ever did. In fact the car and even the exhaust seems to be original. The timing is set at only 5 degrees, so low that the car runs fine on pump regular unleaded. (A little spark knock at heavy throttle only.) The mixture is just rich enough that there is a little sulphur smell at the initial return to idle, and the car barely has any soot in the tail pipe. (This part drives the other guys in my engineering department nuts.)
Sounds like a replacement cat to me. That is $500 I did not need to spend, especially when upgrading the exhaust to a high performance cat system is not a sure fix if we are not sure that the OEM part will work.
In response to the O2 aftermarket system, I have experience with the CIS systems and these control pressure feedback LAMDA units. If the operation is sluggish, suspect the O2 sensor or location. It must be CLEAN, and located either in the manifold or between the manifold(s) and the cat. It must be HOT to work. The OEM CIS feedback system actually controls from the bleed pressure port so there is virtually no difference in the response of the systems.
I am still open for any alternatives.