What are cats?
In all seriousness, when your car starts getting really down on power, and you can trace it back to a restriction in your exhaust, it's time for them to go. Once you've determined that all systems check out ok, you may want to look at the cats as a suspect if your car starts bogging or dying when backpressure in your exhaust gets too high and causes the engine to stall. Your gas mileage will definitely take a hit as well, since the engine is working harder to try to force exhaust past the restriction.
In all seriousness, when your car starts getting really down on power, and you can trace it back to a restriction in your exhaust, it's time for them to go. Once you've determined that all systems check out ok, you may want to look at the cats as a suspect if your car starts bogging or dying when backpressure in your exhaust gets too high and causes the engine to stall. Your gas mileage will definitely take a hit as well, since the engine is working harder to try to force exhaust past the restriction.