I received the dongle from Australia this past Monday, it was easy to install, and I've done two rides since. Both rides were in cold and windy conditions so I consider these preliminary results. It was hard to tell at times if I reached the cutoff or if it was the wind.
Note that I'm in the US, riding a 2019 US-spec Gain M20i, with a 20 MPH cutoff (32 KMPH).
It appears I'm getting about 1 mph out of the dongle. And the assist just drops off at the point, there's no ramp-down. Looking at the eBikemotion app on my phone, the speed is always 10.4 mph, even when the bike is stopped.
My belief is that there is a failsafe parameter being sent from the hub motor to the controller that the dongle has not accommodated. Probably motor RPM. So the dongle tricks the controller into thinking that the speed is 10.4 mph, but once the motor RPM is exceeded, the controller cuts off the power.
I'm hoping to ride today and tomorrow, and will mount my phone to the handlebar so I can see more accurately where the assist drops off, via the app. And via my legs of course.