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
2005 Pontiac Bonneville GXP
2008 Chevy Impala 3LT 3.9L (9C3)
2000 SSXP (retired) all over the country in a million pieces