Yep, I agree, more than about 25mph not only upsets the cops, but cars aren't sure how to react. Too often, when they see a bicycle, they judge it's speed as lower.
I've had countless cars cut in front of me at roundabouts and intersections, because they thought they had enough time.
On the open roads, though, I sit on 35-40mph.
From what I've read here, the 48cc engines rev higher, but the 66cc has more overall power. ie The 66cc would rev higher up hills.
Apparently, the engines' bottom-ends are balanced for the 48cc top-end, hence the higher revs in the 48.