Have heard the Blitz Nur system on a S14 and they sound really nice! Haven't looked into flow between different set ups though.
Cops don't appear to read decibels in tas but on rare occasion that they set up a inspection station they do get you to rev your car. But in saying that my mate had to rev his standard celica to 3k but my s13 at the time with rims, bodykit, and in undercoat was expected to be revved to 5k. Moral of the story, if your carlooks modified they will probably put more pressure on you but they don't really have a clue.