I used some of the Flex track stuff on a few commercial build outs for curved flowing walls. Don't remember if it was this exact brand. Stuff work great, holds well and saves tons of time. As Taos mentioned, it is very pricey. Drywall supply houses around here carry it. Never seen it at HD.
Very nice work. Whenever I do stairs I talk customers into the larger, platform type 1st and/or 2nd steps if space allows. It really makes the base of the stairs more dramatic.