There are so many ways to go about this conversation. You’re most in luck if this is new construction. Then i would say to apply it on plywood of floor and cover again with another plywood.
if you do it from the ceiling, it would probably help but not as much because of all the light fixtures, vents… that would go thru it.
I think a double floor is a waste of money.
green glue on the wall between layers of sheetrock helps alot although not perfect.
I don’t see any benefits of the foam. I don’t remember the name of company that did the work. sorry.