i have been threatened to have my car impounded/boot put on it a couple of times. in my case it was valid cos i was driving like a w4nker. It certainly has got the message accross to me (apart from the first time obviously).... i really cant do without my car, even though i only put about 20km a week on it if im lucky.
IMO police do need the ability to impound cars, i think it is a valid punishment, as long as there is a valid reason.
I'm not well versed in the hoon laws... but my experience with them has been a good one