A couple of thoughts Paul.
1) I've ridden the Interstate on my 250 scooter at 62 to 64 mph and get passed like I'm standing still BUT I also drive the interstate in my car with the GPS and cruise set at 66 mph and get passed like I'm standing still. Even on 55 mph sections.
2) Two weeks ago I took the 750 Kawasaki on a long ride in one night.
http://twowheeler.yolasite.com/index/the-epic-voyage-of-jimmy
Guess what? Even on a high powered machine I was passed at 70 mph like I was standing still.
I totally agree with you on wind buffeting from Trucks. Even on my 400 pound scooter the blast of a passing truck can move me over. Not so, even on the Honda Rebel. That's not to mention the effect of cross and head winds from Mother Nature. Climbing a steep hill into a headwind can slow me down on the scooter by several miles per hour.
I've ridden the Interstate on a Chinese 150 for short spells but I don't make a habit of it.
But I agree. The Interstate is no place to be on a 150cc scooter but then a Yamaha SMax seems to handle it so Horsepower is an important factor.