umm i do believe if a cat. is clogged or almost clogged it will create more backpressure which in turn the car would make the car either dye or idel high situation dependant. now the rotten egg smell that he states is coming from his exhaust is usually the inners of the cat. getting really hot and basically cooking itself.

i do however agree not to go buy on yet and that was why i said get rid of it and run a straight pipe.