What year is your bike? I ask because all Rad Bicycles actually come with the same motor. However, they are spec'd differently for their respective markets via the controller.
Furthermore, Bolton ebikes point out that the motor is in itself not a true 500 and sells an upgrade kit. The latter is a larger motor with upgraded controller that, if I am not mistaken, makes your bike closer to a nominal 1000 watt. Check out his YouTube channel. He is actually selling an upgraded RadRover/Rhino that hS the same off the line acceleration that you would expect from a SurRon.