The point is that most really loud cars are made to be loud. And usually equipped with a way to quickly undo that, if needed.

I've seen a TV report on some policemen hunting "tuned" cars. They were following an obnoxiously loud car in an unmarked police car, stopped them, and took measurements - suddenly, the car was "just normal". But they knew what they heard, and the measurements they had taken from a distance had been way louder than the measurements taken at a defined distance from the exhaust, so they impounded the car for further investigation. And found a switch in the glove compartment that changed the car from "normal" to "loud".

You've got to catch them red-handed. As long as they can disable or just quickly undo something like that before a MOT , it won't get a single idiot and his car off the road.

People would just remove/silence the mods for the MOT and replace them as soon as they pass.

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.