I like to use the Sport Rider numbers because they are available online. They are pretty conservative on top speed. I don't think they ever tested a 600 that would do 160. The new ZX-6R will probably change that slightly.
Yes, it will go up slowly as you reach top speed. Also, aerodynamics plays a big part so as you adjust your riding position and get everything tucked in your speed will go up a little. Where I live there are lots of roads with little or no traffic, straight as an arrow just beggin'.
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