No power is defenetly a sign of a plugged cat. A lot of times you can check for a plugged cat by measuring temperature at the cat. If it's plugged it's going to get real hot, and sometimes it will even glow bright red after driving for a while if it's plugged enough. You can also do a vacuum check to verify a plugged exhaust. I think if you got a plugged exhaust and you check manifold vacuum your supposed to see a steady drop to zero, i've never succesfully seen this happen so I don't know if you are supposed to give it a little gas and hold it and see if the vacuum starts drifting down or if it's just supposed to do it at idle. Usually exhaust shops that do catylitic converter replacement know how to check with a temperature gun to see if you got a plugged cat. That wouldn't be a bad place to start.
Usually when you hear a rattling noise from the cat it's a good sign that it is coming apart.