I think 267 kph or 268 kph is the real V-max for a 1M. It was posted way back when the 1M's first came out - even though "officially" they are limited to "250kph" per the gentleman's agreement.
Btw... I too got into a few "speed contests" on the way back from Essen/Techno Classic. The best was when I "walked" a e92 M3 to my V-max of 280 kph(gps).