Question #1: Did it power out, or did it rev out? My 2016 Honda Rancher will pull right up to the rev limiter in 5th gear, and you can hear when you hit it, it starts to cut out and will not hit 51 mph. Mine will do exactly 50 miles per hour.
Any chance you had the transmission in LOW range?
That's odd. I have a hard time believing it doesn't have enough power to hit top speed on a flat surface. My Rancher has no problem powering up to top speed. I know the Rubi is 70 pounds heavier and probably has more parasitic power losses, but it also has more ccs. Even before mine was broken in, under 3/4 throttle it would hit 45-46mph. I didn't full-throttle it until after 100 miles.
On my 2016 DCT AUTO Rancher it feels exactly the same in ESP or in AUTO and it pulls up to the exact same top speed. In both ESP and AUTO it hits the rev limiter. Would the Rubicons be different? Here's a video of mine doing top speed in ESP and in AUTO:
