Simple room-in-room vocal booth questions...

Long time lurker, first time poster. Howdy! :) I'm in the process of building a vocal/OD booth into my production suite. The booth will be built *tightly* into one side of the outer room of the 2-room office space. Acoustically, I'm trying to isolate the booth from both control room bleed and ambient noise from the hallway and through the shared walls. First question: 3 of the walls of the booth will be constructed 4-6" from the existing walls (it's a small outer room) to maximize the booth's size. I had planned to simply affix 2 layers of drywall to one side of the 2x4 frame, making the resulting system (including the existing wall) 3-leaf. Would it be more advantageous to put the drywall on the inside or outside of the frame? That is, does the additional 3.5" air gap from the 2x4 frame (if the drywall is internal) buy me significantly more STC? Is this likely compensated for (in the external-drywall case) if I fill the frames with 703, which could double as internal acoustic treatment? Second question: I've not been able to find much info on how to best port these sorts of enclosures for cable & power runs. Are there guidelines for doing this without compromising the isolation too much? Thanks for your help!