According to the Speedbox manual the GPS signal will override other signals - VSS or CAN. So, if GPS is available the speed will be taken from that. Only when GPS is not available (before satellite lock, in tunnels, etc) the speed is taken from vehicle sensors.
During a burnout with good GPS signal the speedometer will actually show zero if the car is not moving.
It seems to be a very cleverly designed piece of hardware. I like it a lot!