I hit 64 in a friends XP on sand highway going from Glamis to Gordons. My guess is the Mav will be governed at a certain top speed, which is fine by me, 70mph is plenty fast in the sand, you're never going to hit that in the dunes anyways, just on the sand hwy.
The speed in the sand is a tricky thing because it depends on many factors, type of tires, air pressure, type of sand (wet or dry) and where the run occurs. When my commander was stock it ran about 62 to 66 mphs down the drags or Gecko with 27x12 skat trak sand tires at 7 psi. My stock commander was always much faster or quicker than my buddy's commander cause of the right skat trak tires I used vs the sand tires he uses. Once I installed an improved intake system and a larger TB (60mm) it ran 72 to 74 mph but got there much faster. I bet the Maverick with the right tires, air pressure and the stock air box replaced with K/N style filter will easily do the same.