The way i see it is there is no way to prove you gutted your cat unless they actually see you do it and fit it to your car. Seeing as though they deteriorate over time, and fall apart, you can just plead (dunno if thats spelt correctly) ingorance and your free. If you have a piped cat, again (like any offence) you can plead ignroance and say the exhaust has been untouched since you bought it, and you never knew. All they can really make you do is fit a new one. If you have a gutted/piped cat, just keep a working stocker and if you are sent to the pits, swap them over... problem solved.
Also in regards to the environment comment, i agree, and im sure its not the best.... however in the end it is a performance car, not overly economical even from the factory, and to be honest i can hardly see it making a real difference. If you think it does, then go tell everyone who farts, produces a smokey burnout, or smokes cigarettes to stop..