The fastest the Veyron can possibly go with draft, and ideal tuning is 287. Any higher, and people are lying, nothing more to it.
My buddy and I have taken our two Formula GT's on SSR7 and followed a Veyron to get us up to speed. We minimized the downforce and maximized the gearing. With the Veyron about 100 feet ahead in the tunnel, we hammered the throttle and got to the redline at 292 mph. He recently got an X1, and we did the same thing, (Bugatti, FGT, X1) and I can't remember what he got, but it was higher than the Formula GT.