Thanks for the heads up - apart from anything else, if these guys are like the ones Col posted, it's a bit of a worry. If they're willing to pretend to be cops, they're obviously willing to break the law - and who knows, they might be willing to hurt you or damage your car unless you apologise profusely, give your "hoon" car to charity, buy a Prius and make it your life's mission to rid their neighbourhood of hoons.
Personally, as has been mentioned, if they do pull you over I'd be making a phone call to the local station - if they turn out not to be real cops, I'd be telling them to stick their vigilante crap where it fits and ensuring that the REAL cops were given as much info as possible. If they DO turn out to be real cops, you probably won't have anything to worry about.
rofl
If there's a huge problem in the area I can understand their motives but geez, how stupid are these people?