Soffit mounting is OK but the speaker should protrude from the soffit at least an inch or so - the more the better - otherwise imaging can be affected by the surface of the soffit. In studios the soffit is always completely dead acoustically and the walls are built with concrete - wood will resonate and cause a real mid-bass problem. All cavities behind the soffit must be filled wilt rockwool or other type of damping material.
The other issue with soffits is that energy below about 80Hz will get a 3dB boost in output compared to stand mounting.
The IB1's can be soffit mounted.