Something tells me that the probability of reading about what someone has done to their car, and spotting that exact car on the road, and remembering everything illegal about it is sooooo tiny, it's just not funny. And thats _if_ they post something identifiable about the car, as well as the illegal mods.
If I said my car is 5mm off the road, has a gutted cat, and produces 5000rwkW, and I'm on my P's and drive a Silver EA... wow, that cuts it down soooo much it's not funny, given that I only see 2-3 other Silver EA P-plater drivers a week!
I guess if it's an import, and theres a few people with these imports in a specific area (eg. Knox) AND they're all dumb enough to say where they are and what illegal mods they have.. well, **** it, they're just too retarded anyway
In short, unless you post personal details, and a list of illegal mods / activities on the forums, the cops wouldn't have much to go on anyway.