No issues at all on the freeway at 65-75. The CVT moves through its gears up until 75mph (RPMs stay near peak torque) once you cross 75mph the CVT is at its max gear and you need to start using RPMs to go faster. Even cursing at 80mph isnt an issue for the scooter, it more the wind coming at you at those speeds isn't that comfortable, it could really use a larger windscreen for highway travel.
I'm 6 foot 210lbs and there is plenty of power to get up to 85mph easily, after that acceleration drops off quickly but I have been up at 95 so far and there wasnt much left.
I used to have a C600 Sport and there is certainly better wind protection at highway speeds, but it is also 100lbs heavier and not the greatest for city/around town stuff.
If you mainly ride on the highway the larger scooter will better for sure. The C400 is a good compromise as it's light enough for daily around town stuff. Yes it's fine on the highway but it's certainly not it's sweet spot.