hmmm, doing 120 at 9K RPM???
Top gear ratio is 0.913 in stock GS150. That means to achieve "true" 120Km/h , you 'll need to be at 11K RPM.
probably serious error in speedometer.
Even if it has 3.50 tyre at rear, RPM can't be that much less.
Stock GS150 will give
98Km/h speed @9000rpm,
109Km/h speed @10000RPM,
120Km/h speed @11000RPM,
and you are in luck if you achieve 12000 RPM in 5th gear, it will top out at 131Km/h speed
most probable reason that you got this speed at 11000 RPM is that you had accurate meter. It seems accurate theoratically also.
That being said, though i have not pushed my bike (GS150) much yet, but i have noticed speedometer error at low speeds which probably means error is gonna be much higher at high speeds.
Alsons is pretty much crap now a days, even chinese meters have less error than my Junoon's original Alsons meter.