But you are not likely to see this enforced outside of rich quiet neighborhoods where police don't have anything better to attend to. Culturally, sentiment in the US does not want the law coming down hard on drivers and their vehicles. For instance, most US states do not even require vehicles to be safety inspected. And in urban areas the police have worse infractions to be going after.
It would be bad only if it didn't result in decrease of loud vehicles. If they run the dyno and impound vehicle, it means that this loud vehicle won't be coming back this street for some time.
The only provable noise polluter in that case is the police.
I'm sure they could also put my car on a dyno and prove it could go 100mph on the 30mph street I was on too.
One assumes this process is generally reserved for vehicles whose driving has earned them a dyno inspection, as it would induce mockery were they to e.g. dyno an old beaten-up Geo Metro without cause, and that the fines are directed towards either the vehicle owner and/or manufacturer, as circumstances require, as the responsible parties for its mechanical-design and/or aftermarket-modded violations of the law. (I am not your lawyer, this is not legal advice.)
https://www.buzzfeednews.com/article/salvadorhernandez/leade...
Actually, you can argue it and it will work.
Failing that test is expensive, and you don't see your car again for a few weeks. They revoke your permission to operate the vehicle on public roads, so in addition to the fine, you face fees (towing, storage of car in evidence locker, towing your car to a shop after getting it back in order to make it street legal, then redoing the road-worthiness certification).