There are two "official" versions of the BMW M6:
One is limited to 250 kph, the other one is limited to 305 kph. The sports package with the 305 kph limit has to be ordered as an extra option, it costs around 2500 Euro in Germany (I have to look up the precise price tag) and also includes a driver's ed training course (which is NOT mandatory, it can be skipped but it costs still the same).