hard to say. i have heard of them going at low and high mileage, just depends. quick way to figure out if yours is bad is, when you start the car and it is just sitting there idling do you hear a metalic clinging and clanging? if so then the guts of the cat have come loose and it needs to be replaced if not and don't have an engine light on throwing the code for the cat then yours is fine.
the best part about the 3" on is that when you put headers on it is the same size as the down pipe of them so easy weld and you are good to go

My Mercury has 340000km and my Buick has 200000km, both with the original cats.

A friend of mine who owns an exhaust biz cut my old cat out and installed a magnaflow.

$150 bucks total.

Here in IL you have to pass emissions.

C