I agree with the others that your going to have to remove it to investigate. I'm not sure what kind of cat is under there, it may be difficult to visually see if it's ok. Driving it with a test pipe would offer the awnswer though.
There's alot of aftermarket hi-performance cats on the market now that flow well and don't cost alot. Short of cams, I don't see why the newer ones would bother anyone to run. I use a three way on a car that was built long before three ways existed. It's a nice cat, it can see right through the honeycomb - flows pretty good, I have performance mods, and I still pass smog. Given a choice, I wouldn't run one, but I'm more comfortable with it, and following the rules with my daily driver. My T/A is another story though - it's probably going to be a fire hazard with my cam selection.
As far as telling you the symptoms of a clogged cat - I think there's to many variables; how clogged it is, and the type of motor you got. When mine clogged I had more power to 2k then power would just drop, you could feel the motor struggle, and didn't have the power to spin to redline in any gear but first. That was a really clogged cat.
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Jon B 
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