I have been followed home from willoughby to crows nest a few times.. Its allways a bit sketchy..
Alot of the highway patrol guys have a real attitude problem, its like they have it in their head that we modify our cars as a personal vandetta against them.
My suggestion is dont panic, just go get the exhaust tested and engineered. If its to loud, get a silencer or something. If not then take it to court with the correct documentation and you will get off.
Whilst some of the police officers act like immature brats, you still have the law on your side if your not doing the wrong thing. Just prove to the people that its legal, and you wont have any issues.
When I finally finish my cars build up I want to have everything engineered so if a police officer wants to be silly, I can show them documentation.