Is there a way of telling if it is on the verge of getting fully plugged up? I would guess that it would do some damaged if it got super hot.

One of the first sign is a rattling from the front two converters at idle and exhaust pipe very very hot before the main converter and cold after the main(center) converter, exhaust note will get softer no more rumble, and motor feels down on power, not missing or stalling just low power out put.
David

As your cat gets clogged you loose power higher in the RPM range, with more throttle first, just like when an engine lacks fuel...but smooth. No misfiring.

Also, you'll get a LOT of intake/induction noise at high loads.

Cat life depends mostly on the tune of the engine. Stock engine/stock tune/properly running, the cats should last at least 150k.

Those damn things are pricey!

rich running cars will plug up quicker so keep your ride tuned right!

I haven't heard of too many plugging up on Vettes.