I don't have much knowledge of in-wall stuff, but you would cut the sheet rock back to middle of the stud on both sides, install the sub, and place the sheetrock back over the sub (except the port and driver - I assume it comes with a grill (usually metal and you can paint).
I would use construction adhesive (Liquid Nails) everywhere I could and especially for the sheet of sheet rock that goes on top of it!
I'm not sure about phase issues, but I suspect you would get a good amount of room gain.
A good in-wall system can make a huge difference in making the best use of space in a small room and it looks slick in any room!