The experience in Germany is, that manufacturers will just engineer around it. You define a testing method and they will make sure their exhausts comply in that specific scenario, but are loud in all other cases. The most egregious example are probably motorcycles. The law limits most to 77dB(A), but in reality many are closer to 90dB or even louder. The government has been trying to define better standards, but we will have to see where that gets us. Real-life measurements are much harder to fake and probably more impactful.