Here's a
You-Tube video of a 35i that
almost pegs the speedo at 256 kph.
Considering that most X3 speedos read about 1.5% high, that still equates to 156mph. Top speed is achieved in 6th gear and it takes a full minute to get there.
For a plot of the acceleration of this 35i, (derived from the video) see:
35i acceleration graph.
From the acceleration graph, the shift points and the top speed, it appears that this particular 35i has been subject to a mild "tune" and probably has had the limiter removed.