I've ran 51.9 mph with my E-maxx using 12 cells and a Hacker 7XL. The truck is very light and was using 24/51 gearing on stock diffs.
A trucks weight will make a difference. The light truck can pull a much high gear and is faster.
Those small basic motors are fine for the E-maxx if ran as a dual system. But one BIG single is the way to go.