From what I recall in a thread about the speed limiter and ECUs in previous generations of TDIs is that the max speed can be tweaked somewhat using VCDS (VAG-COM). It's thru one of the adapation channels available. The gotcha is it can only be adjusted DOWNWARD, not upward. So if you adjust it downward at all, you can't raise it back up. If you adjust it too far down and it cripples driving at "normal" highway speeds, you still can't raise it back up. New ECU time. (Or maybe reflash it with a tune might fix it?). IIRC, in the Mk4 TDIs in the USA, the limit is set at 190km/h (~ 118MPH).
Like others said, it's a combination of the engine running out of breath and hitting the electronic limit.
Good luck.