One of the main problems with a lifting tail is that the tail is in the wing's downwash, this means the tail's induced drag is way out of proportion to the lift it generates. A conventional negatively lifting tail, on the other hand, is surfing on the downwash of the main wing and thus it's induced drag is low. Instead of giving air that has already been accelerated downward even more energy, it can recover some of that energy by decelerating that air. If you want an efficient lifting tail, it would have to be outboard of the wingspan somehow. There is a reason that migrating geese don't fly right behind each other in a single file.