1. Shifting is not difficult but it can be painstaking in a high density traffic area where stop and go traffic is constant (there is a post somewhere I did that list top 10 worst traffic in USA). Many cyclists opt for the larger CC scooters here due to that fact. Then again there are tons of people also on motorcycles that just “shift” their way through traffic like me.
2 – 4. There are larger scooters, primarily 500cc’s, that will do great on the freeway. I have seen several in Houston, but of course they have to be larger than the 150cc engines. Typically there are 300-500cc scooters around.