I think it was Lusche that wrote that the range is shortened when driving forward because the "wind" causes the rocket to weathervane downward into a lower, shorter trajectory. Backing up has the opposite effect and the rocket will go farther. I believe I tried this at the time and found it to be true.
Again... if you're speed is constant and the angle of departure is the same then if there is any difference it won't be noticeable by us commoners. You can test the theory by using the low angle and sitting on the end of a run way. Fire 1, then "measure". There is your control. Then get to 10 mph, fire a rocket and measure. Etc. Film it. Then form your own opinion.
Or... use the laws already in place.