I hope not! Thats what I would have prefered!
The 6mph ones look identical, inside and out and read the same internal resistance on my fluke, and the only difference is gearing. They draw trhe same amps stalled too at 2V on my bench supply. So the motors are the same electrically. At least in performance. In spite of the 8mph ones being marked up as 550 watts instead of 450. They MAY be longer? I didnt measure, to lose more heat? Hence the higher cont rating. So the 6mph ones have about 25% more torque, and less current and so battery drain at every speed. So most users would definitely prefer them. Theres even warnings on the prescrition forms about this with 8mph motors for heavy users. And an insistance on both a 10A charger and 120A controller with the taller geared 8mph ones.