Everyone who has seen these things says they appear restrictive -- so, Mike Wilson provided some pictures so you can all see if they look restrictive.  Mike's old cats had been cooked, so he replaced them with the later versions that lacked the honeycomb inserts.  "Here is a picture side-by-side of my old unit that melted and the new model I just installed.  Man, I can see the advantage of getting the models without those restrictive inserts!  Part number is: CAC-7959."

Now, besides being restrictive, these things are also obviously good filters.  Bob Gallivan sent this pic:

Besides the partial pluggage, also note that the honeycomb insert is apparently formed by wrapping wrinkly sheet metal around a metal tube center -- and that the metal tube center is missing from the downpipe on the right.  It's probably rattling around in the exhaust system somewhere.

Wilson:  "Now this is really interesting!  On a 1990 XJ-S, the intermediate pipe (on the left) is supposed to have a larger pipe to allow it to slide over the downpipe.  However, you will note that both pipes are the same size, 2 1/8".  So, I had to take the intermdiate catalytic muffler to my local exhaust shop and have them enlarge it.  This pipe also came in a box with a part number of CBC-8213."

"Both units are "aftermarket" from XK's Unlimited."