Perpendicular / Sideways Studs, Thin Partition Walls, Math and Soundproofing

This is my first post, please be kind. :) When you're tight on space and it doesn't take long to consider turning the 2x4's sideways so that the studs are now rotated 90-degrees and perpendicular to the adjacent wall--you gain 2 precious additional inches of room or closet space doing this. So when is this space saving/gaining thin-walled construction a good idea and more importantly when is it bad? Walls with doors seems to be a no-go, and certainly exterior or any interior load bearing walls are clearly out (against code as well). But I was not finding any rules of thumb that helped to decide when thin-wall construction can work well--the only obvious choice would be for closets, and beyond that I didn't know. if anyone knows of a good source for some rules of thumb then please reply. The thin wall (sideway studs) will pass more noise through them, not because the sheetrock is closer together, it's because the studs are not as rigid sideways and you have more direct connection of the stud to the sheetrock; if you can add 1/4" of styrofoam or other soundproofing material then it would help. If you really need the thinest wall possible and it must be soundproof, then consider draping plastic inside and then fill it up with DRY play sand; to dry the play sand you have to spread it out on the floor with plastic and indirectly blow the air around with fans, and rack the sand a few times a day for a few days, but you'll have maximum soundproofing with you thin walls! You fill the walls by laying sheetrock sideways, and filling it up 4-feet, then 1 or 2 feet at the top. Filling to the top is not as bad as you think, especially the last foot, and you're already a stubborn determined DIYer who demands those 2 extra inches while refusing to give up an inch (or would that be decibel) on soundproofing! If you want decent soundproofing, don't want to spend big bucks on materials, fear the thought of racking sand on the floor for days, then cut a sheet of firerock almost as big as the inside of your stud, then drill a hole in each corner, about 6" inward left/right and about 24" from the bottom or top; place a rubber grommet inside each hole and run a wire in the bottom holes and feed up to the top hole, loop through that and tie onto a hook at the top of the middle of the stud. This extra mass will help stop sound waves. and will not vibrate with the studs of the wall. If you like you can seal around the edges with caulking and that will also not pass sound waves. The very best soundproofing methods require making walls thicker, so I've limited my focus...