I think only the cannard style planes can use positive lift on the tail. Stability is caused by the negative lift of the tail plane allowing the nose to drop when slowing down and up when speeding up. Yes, a rearward CG increases efficiency by requiring less negative lift of the tailplane. Aircraft use fuel transfer aft to accomplish this increasing fuel economy. I did it in the older Lear Jets 35 years ago. Fighter jets use computers to not require much or any tailplane negative lift.