BIAB is just another method to mash your grains. It typically uses all the water in the mash with no sparge but many of us have reduced the amount of water and added a sparge step for the additional sugars we can extract that way. BIAB can use a finer milling without problems draining the mash but it doesn't require it. Without the finer milling you should be getting similar effeciency as a conventional mash tun.