Cats just don't go bad. If your car is running too rich, the extra fuel can end up in the cat and cause it to melt into a rock form. Banging on the cat you should hear a raddle sound or have blockage in your exhaust causing or car to run poorly. Replacing the cat will not fix your problem, it will only happen again.
What I would do is take the Viper to Auto Zone and get the code number checked. The code will give you direction where to look and you can come back here and get advice. You can also start the car and check the pressure coming out of the exhaust pipe and compare each pipe.
Any mechanic will know that cats just don't fail, it's something else that causes it. I think the dealer was lying about the cat being bad. They should have discovered another problem that caused the cat to fail. They just wanted to sell you something that you don't need. I always like the part when they tell people that they can only install parts that they order. Never trust what a mechanic tells you, especially if it is going to cost a lot for a repair.
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