The 3 shaft boxes with the 360 motors should get you really close to scale 50 mph. I got 47 in the 99A, but that's with bushings on everything except the axle shafts. The new boxes are much smoother so that might be the last little bit needed to get up over 50. And the RC M1 may be a bit lighter than the 99A. This will give you some idea of the speed difference between stock 380 motors and the 360s. Two identical tanks, one with 380s, one with 360s, in a drag race. I also did videos of the time trials of the 360s in the 99A.
https://www.youtube.com/watch?v=gzW9U5pkMiA