I don't have much insulation above the light boxes, since my floor joists are but 7.5" tall and my backer boxes eat up almost all of that height.
The Bazz 100 in those photos of mine, are remodel cans. The single layer of drywall was also about the clips that secure the cans, as they wouldn't work with 2
layers of 5/8" drywall.
I doubt the insulation is doing much anyways, save for preventing the floor joist area from becoming a resonate cavity. (Insulation will eat up high frequencies,
but do nothing to stop long bass waves.)
Don't forget to caulk the holes where the electrical wiring comes in to seal the box.
I would wager your door leaking sound will be an issue long before properly sealed backer boxes ever will. They are simple to make, inexpensive (Big's version far
more mine), and work well. I will double down with Big's design next home theater build, when I retire and relocate. Kind of hard to argue with good, cheap,