Cool. I am a big fan of using rock wool insulation. I used rock wool in the ceiling and interior walls and 6” regular fiberglass insulation in the exterior walls of my basement listening room. I used sheetrock but floated the ceiling sheets on aluminum brackets to reduce transfer of sound to rooms above. The room has a concrete floor poored over tons of crushed rock, so quite different than a wood floor.
The room is pretty neutral with only a few nodes in the bass region. With a bookshelf running the full length of one side wall and only three smallish windows on the back wall, the room has almost no reflections and treble is a tad reticent below about 50 Decibles. Above that volume, sound is outstanding, with only the reverb of the original recording space presented as well as the speakers can.
I would try setting up your gear with nothing covering the insulation and trying it, and then try tacking up at least half of the space with dry wall, see what sounds better and go all in with that. Agreeing with Sisyphus51, I am wondering if you really want to approach a full “anechoic” listening space. Also, are there any health issues associated with exposure to unsealed rock wool? Finally, I find having a hard floor with “dead” walls is a good combination, try the tiles suggested above as well.
kn