[JL8Jeff]

It might not be the cats as much as the O2's are giving bad readings which screw up the computer mixture. The factory cats are made to heat up as much as possible to burn off emission's so the temps don't really tell you too much. That being said, one of my factory cats smelled funny from the day I got the car with 33K miles on it so I'm sure there was some buildup in there. The PCV setup on the LS1 allows the engine to suck in oil and that will cause buildup everywhere which will throw things off over time. I'd try the O2's first and if that doesn't make a difference, then go after the cats or install headers.