Soffit mounting is common in studios. You should see no problems as long as there isn't a large amount of room around inside the cavity where the the sub resides that would cause resonances - this could introduce peaks and dips in the response. You would get the same re-inforcement from the wall and floor gain as if your sub was not mounted inside the wall. If the soffit was in a corner, you would get reinforcement from 3 sides instead of two, and more gain.