I've ridden a Speed Pedelec and the experience is a little under whelming.
The motor is no more powerful, the only difference is the cut off speed.
Ebike motors are built for torque, not speed, so the assistance over 15.5mph is relatively small.
It does make a difference, but I doubt many cyclists would have the power to cruise on one at 25mph on the flat.
No extra speed to be had on climbs, unless you could do the climb at more than 15.5mph on a legal ebike.
That's is what I wanted to know, no absolute benefit.
Ive seen 1500W ebike s and they will hit 30mph on the flat with little effort from rider