Well as it turns out they are only doing 1/8 mile drags, about 200 meters.
As for only reaching 160KM/h. The bike has the wrong model CDI for the engine that is in it, has a 57 tooth rear sprocket, 140 section rear tyre, and the engine is over 20 years old, though with new rings and gaskets.
Once I replace the CDI with the correct unit, put on a 55 tooth rear sprocket, and a 130 section rear tyre, I'll have another go.
You can see this video where the rider gets it to 190KM/h plus, but it's an earlier non-restricted engine.
Link to original page on YouTube.