Do not gut your cat!! I would like my kid to grow up without a gas mask.Also it creates backpressure by stagnating the air flow.Actually I don't quite understand,you said you replaced your cat with straight pipe,right?So what are you thinking about ugtting.Basically,take it back to the muffler shop and make them redo it,and watch them to make sure they do it right.Mention negligence and replacing melted bumper and you will surely get it done right.