While posts on a forum cant be used directly for prosecution (at a guess), I'd imagine they can be used as evidence to form a character profile of a defendant. If they are freqently admitting to driving stupidly then I guess a judge/jury would look very dimly when the defendant stands up in court to say they couldnt have avoided said accident.
Of course doing 90 down a motorway most of the time is no problem, but it is illegal and shouldnt be condoned on an open forum.
What I dont get is why people want to post how fast they have gone? In cars such as those with large engines with plenty of power, especially with modern chasis, it really isnt more than a case of holding your foot down for a matter of seconds before you can do silly speeds. No skill required. I dont get what there is to show off about