You're reaching a bit to think that a cop is going to pull you over and hassle you about an aftermarket exhaust that is the same or quieter than your stock one. By reaching, I mean I have never heard of it happening. Ever.

However, should this happen and you're written a ticket for improper equipment when your equipment is indeed not improper you should simply challenge the ticket and it will be thrown out. The police know this as well, which is why you likely won't be bothered if you're in the right.