Weight has nothing to do with top speed.
From everything I've learned in physics, it does have some bearing.
If you're telling me that I could put a whole thing of lead in the passenger seat of a Veyron, then get to 253 MPH, I don't see how that's possible. While it may get there theoretically, there's no track in the world where they would have enough straight line distance to get the theoretically heavier car up to that speed. IE: The Pur Sang, at Ehra Lessen, could get to 253 MPH faster, and in less distance, thereby providing more distance for going faster.
PS: On my formula sheet for physics, mass is in just about every equation for solving for Vf.
EDIT: Just realized that I may not have been the most eloquent in what I said above. Let me rephrase it:
You've got 8 miles of straight, flat, smooth road. On the left is a Bugatti Veyron. Completely stock, completely normal. In the middle is another Veyron. Absolutely identical to the first, except there's a bunch of lead in the passenger area. And on the right is a Veyron Pur Sang. All 3 set off at the same time. The one that will get to 253 MPH first will be the Pur Sang, followed by the normal one, followed by the one that's been weighed down (if it can get there in 8 miles - I haven't bothered to calculate it). Since the Pur Sang has gone to 253 MPH first, we assume that it still has some track left, and the driver continues to floor it (assuming the speed limiter has been removed, he has enough fuel, tyres are intact, etc). He will be going the fastest by the end of the track. Theoretically, the weighed down Veyron should get to the same top speed, it'll just take much longer and you'll need a lot more track. Since the normal Veyron only really
just gets to top speed at Ehra Lessen, I doubt there is a facility in the world that you could get to the same speed in one that's much heavier. By the same token, you should need less track to get to the same speed in the Pur Sang. You'll have more track left over to play with, then.