I thought about your post last night while racing and took a closer look at the X1's together in my garage. I know I had seen the HP figures of all four listed as the same in either the official GT website or in game in the last week. Now there are 3 different HP figures for the 4 cars.
So I tested the X2011 and got an fairly easy 351 mph on the speedo and it saved as 350 mph in the stats section. That's exactly what happened with the Veyron. It went to 351 mph and the stats said 350. Except with the X2011, the way it was increasing speed I thought for sure it would pass 351/350 mph, but it didn't. This leads me to believe that the game speed is locked at 351/350 mph. (350 mph = 563 Km/h)
And Chryzsh, I think your city, Bergen, Norway, may be the prettiest town on Earth.