worlds smallest studio

[TexRoadkill]

How much sound would leak out (up stairs) if i didn't add the sheetrock?

I guess i'm a little unclear on the dynamics of absorbing sound and
keeping from leaking outside and how they interact.

Sound reduction requires three things. Air tight seal for the highs, mass for the bass and dampening for the vibrations. If you skimp on any of these things you can make your sound reduction expense all for nothing.

The problem is the better a job you do at keeping sound from escaping then the better a job you need to do at absorbing sound in the room. Otherwise the sound waves just bounce around until they run out of energy.