I’ve been riding a Class III Riese and Muller Roadster with no throttle for about two months now. Most of my commute is pretty flat, and there are places that I can et to the top speed, but since I'm not a very strong rider I find it difficult to reach and maintain 28 mph on flat ground without a decent tailwind.
On top of that, I find that for commuting through city streets 20 mph is close to the highest speed I'm comfortable going with other bikers and vehicle traffic nearby, so having the extra 8 mph of speed doesn't seem to make much of a difference in my commute. Sometimes I wonder if the extra torque the active line motor has might have helped me get started from a stop when I forget to downshift.
However, I find that I can easily reach 20 mph with little effort on high pedal assist levels and think that having the bike cut assist at that point would be jarring, so the extra 8 mph of room probably helps me feel like I'm having a smoother ride. I also like going fast on some empty roads that I've found while riding around on the weekends.
On the subject of legality, I think that as long as you don't ride the bike irresponsibly you should be safe from police attention, but if you get into an accident you probably won't be given the benefit of the doubt with an illegal bike. I'd personally avoid it, but I'm the cautious type.
I have a mid-drive Bosch (Active Line Plus) and don't notice any motor drag when I pedal faster than the assist limit on long gentle downhills. I do notice motor drag when I pedal with assist off, though, so I suspect it is more noticeable on acceleration at lower speeds rather than on maintaining a gravity-assisted speed.
This has been my experience with the Performance Line Speed motor on my bike. Turning off pedal assist at low speed makes me feel like I'm dragging a bit while pedaling, but I don't feel that while going downhill and pedaling over 30 mph.