It has been my experience that a lot of people who have fast cars these days have absolutely no idea how to drive them. I mean, I don't know how you could fail to drive a Porsche 911 (or 959 or whatever it is these days) fast, but I've easily beat numerous ones from stop lights in my stock '87 BMW 325e. And they were obviously trying to race.

On a legal point, I do this a lot, but when I get to the speed limit (or at least how fast I'd be driving on that road under normal conditions) I stop accelerating. Not quite as illegal as totally racing, plus, I think, it makes the other guy even more pissed off that you've obviously stopped trying and they can no longer beat you.