On a technical point, the speed cut off is by means of a magnet on the back wheel and a sensor on the chain stay.
Tinkering with the gears won't have any impact, the motor will still cut once the sensor reads 15.5mph.
It is possible to effectively remove the speed restriction by fitting a little electronic box which fools the system into thinking the bike is not going as fast as it is.
Sounds good, and it is to a degree.
But the motors are tuned for torque, not speed, so while the motor on a derestricted bike continues to offer assistance above 15.5mph, the amount of assistance is relatively low, and gets lower the faster you pedal the bike.
The manufacturer in the link calls their box a 'tuning' device.
That's misleading because it doesn't alter the motor's power output in any way, all it does is allow the motor to continue to run when the bike is going faster than 15.5mph.
https://www.badassebikes.com/en/