When you say "half walls" do you mean the ceiling slopes down to a 4-5 ft wall on two sides, or the game room is a loft open on two sides to a larger space?
If the latter, then you can realize the benefits that a 15" sub has. Lower bass cutoff and greater efficiency. Two small sub's can be "tuned" (by moving them around) to reduce room nodes in the low frequencies, and for most people with average size rooms (and resonant 5/8" sheetrock on 2x4 studs) cleaning up the low bass is a big problem.
Depending on your main speakers there could be more of a gap in acoustic output between a 15" sub and the mains. If your taste runs to action movies this won't be an issue. If you listen to, say, acoustic jazz, then the double bass or Fender bass will sound "wrong".