That is pretty close. I would be looking at the head gasket or cracked head as the root of the problem from reading your other threads (you mentioned oil getting pushed out of the dipstick tube I think).

As far as the cat goes, if you think the XJ isn't running up to par, it would definitely be something to look into changing as you do have alot of mileage on it but if your jeep is still driveable (or at least was prior to the overheating issues), it shouldn't be restricted enough to create anywhere near the heat needed to overheat the engine.