No experience with the HiFiStay racks here, but I can vouch for the efficacy of concrete floors and ASC's Iso-Wall construction.
I moved my system from our living room (solid wood floor, two layer subfloor on posts and beams) to a music room I built in our large garage on the concrete slab. ASC Iso-Wall construction throughout.
Since I changed a bunch of variables all at once with the move I can't attribute the overall improvement in music to specific elements of construction, but it has been a very large improvement. I've heard the same sort of improvement before though not as large in magnitude, when I moved a system from the first floor (carpet, wood floor on joists) to the basement (poured concrete slab) in another house.
Here's the current setup:
View attachment 162739
Much as I'd love to try out the HiFiStay rack system I haven't worn out my Stillpoints ESS rack yet, so that will have to wait. . .
Steve Z