all of that goes out the window with obd2 cars as they do running tests of the cat. and in order for the light to be turned on, the system has to see a fault twice in a row with the same parameters in order to turn the light on.
with 261k on it, i'd say the cats are pretty much done...
i do not put much faith in products like 'gtp', though. seafoam yes, as a maintenance treatment to remove carbon and such, but not much more than that. there are no 'magic potions' that will make the car run properly if there is a fault with a sensor or other component.