Interesting stuff
I have another formula which is not too bad for some rough calculations:
max topspeed = 45 (seated driver) or 48 (driver nose down) * 3rd root (engine power in hp at crank)
example:
seated driver on a 27 hp bike = 135 km/h max speed.....not far away from the data of a GT !
The factor 45 or 48 includes the average air and rolling resistance and is suitable for some first calculations......