I'm sorry that I can't help with the problem, I've never gone that fast. So far my best speed is 55 but I only have 220 miles on it. I'll try the higher speeds in a little while. At 40 or higher the wind noise is almost unbearable.
I can deduct from this that you have never been driving a naked bike at, for example, 150-160 mph.
DGRDAN:
1. Adding windscreen will IMPROVE a bit your top speed, not decrease it due to improved drag resistance. So, your initial experience with "shaved off" top speed was not correct and was most likely caused by other causes (air density, fuel quality, tire pressure, road incline, temperature, combination of everything, blah blah)
2. Considering that your bike is in fact new, I would check the following to troubleshoot the issue, and most of them are quite easy to check:
1. Electricity: CDI and other related spark-source gadgets (I do now know details of your scoots electric system)
2. Fuel alimentation: to troubleshoot top end performance, I would check the needle and main jet in the carb. Considering that you have injection, you are screwed and will have to rely on proverbially bad skills of maintenance personnel using diagnostic tools (this is why I have injection in simple machines like cheap scoots). Then i would check fuel pump(s) to see that no hose is pinched somewhere so there is not enough fuel when it is needed.
3. Top end performance can be compromised if air filter is clogged, so I would check that too.
4. I would check that exhaust system is unobstructed. This can happen sometimes with 2stroke scoots, not that often with 4stroke.
Considering that you have injection, there might be some bad sensors too, but again, you can check that only with the tool.
So, I would say that you are left to the mercy of service personnel, unfortunately... Keep us posted.
p.s. I would not say that vario can cause such a drop in top speed of a new scoot.