Yes, it would be interesting to find out how your shop determined the car has "plugged cats." Given the cost of catalysts, this might be worth some shoe leather.

Cats have several failure modes. They can degrade chemically, which will usually show up as high emissions when everthing else checks out okay. They can also degrade mechanically (The honeycomb substate begins to disintegrate.), which is often indicated by a rattle. This can also cause high exhaust backpressure, which will result in poor high load performance. The matrix can also get plugged from combustion byproduct contaminants, particularly if leaded fuel is used or the engine has high oil consumption.

I think a lot of catalysts get replaced on little more than a whim, and that costs owners a lot of money, which could be better spent elsewhere. Before I ever replace a catalyst I would need a "preponderance of evidence" that replacement is actually necessary.

Duke