Quattroruote's listed top speed of 327kph was actually a tailwind speed, and AMuS's 321kph was actually a headwind speed —> average = 324kph, right on par with Ferrari's claim.
The 959 had factory upgrade options that allowed it to reach UP TO 339, or 344kph without the catalytic converters. Basically, that is as fast as the Jaguar XJ220 and Bugatti EB110 GT.