You seem to be failing to grasp a simple point yourself. Off-road, a dongle will make virtually no difference. A dongle doesn't change the power one iota. It only removes the speed limit. Uphill, you won't get past 15 mph. Downhill, you'll be freewheeling without the motor. Where would you be able to pedal over 15 mph off road. All the stories of 50 mph are total ball cox. Any bike, including yours, will do 50 mph if you chuck it off Beachy Head.
Look at Soundwave's videos to see the speed he actually travels at with his dongled bike. The display shows about 12 mph, the same as the rest of us. There's absolutely no reason for all this hysteria about dongles. I don't have a dongle on my bike and the speed is restricted to 15.5 mph. Why, because the difference between restricted and unrestricted is very little. Dongles are great if you want to speed up your long commute, because on a flat road you can pedal into the range between 15 and 20 mph. A strong pedaller can probably reach the same sort of speed as your average roadie. That's about the only advantage you get.