I considered doing this, and considered the ABOM fellowship. It's essentially remote fellowship - read and then take a test.
At the end of the day, I think the jobs are somewhat limited and this limits where you can live and practice. You have to be in a large enough area to need a sub-subspecialist, or superspecialist as you put it. 🙂 I may still get that fellowship, as I would be better informed, but I don't think it would change my job much. I don't want to live in a large enough city that it would make sense to do anything besides general endo. My impression is that pay is the same - as rokshana said, it just comes down to how you want to spend your day.