I dont think that riders weight affects the top speed. It is only the acceleration which will be slightly less with heavier rider, although I doubt it would be noticeable without checking by stop watch.
What does affect top speed is air resistance. Crouching down will increase the top speed.
Of course gradient and wind speed/direction has a big effect.
My C15 ss now reaches 60 mph, then slowly creeps up to 70, no more.
Mick