I'm sure you're correct about muckers' and drivers' relative pay scales. "John" didn't work as a mucker, he was a steel driver. The ancestor of the family recalling "John" worked as a mucker.
There is no other evidence that their "John" is John Henry, but how many different men can win all of the steel-driving contests in a particular locale? F. P. Barker, of Birmingham, AL, writing in in the late '20s, place John Henry at Coosa Mountain, AL, in the 1880s and said that John Henry was the "champion of the world with a hammer." This fits with the story about "John."
"...the late-1800's are pretty easy to document." Wow! Have you tried finding documentation of particular ex-slaves in Alabama or Mississippi at that time? If you know how, please give me some tips.
The closest I've gotten is to find a Henry Dabney in Copiah County, MS, in the 1870 U.S. census. That's the right county, where Crystal Springs is. He was born in 1850, making him 37 years old in 1887. I think it possible that he could have been John Henry, but that's as far as I've gotten. I've also found from the 1860 U.S. census slave schedules that there were a number of John-Henry candidates (right sex, age) at Burleigh plantation, owned by Captain Frederick Dabney's uncle Thomas, and two more at Raymond, owned by Captain Dabney's father Augustine, but these records don't give names. I need to find the records of Burleigh Plantation itself and the papers of Thomas, Augustine, and Frederick Dabney, but I suspect that these no longer exist.