A fresh viewpoint:
Q. Why is it that when the CG is moved aft, the airplane is more efficient (albeit less stable)?
A. Because the aft CG means the horizontal stab doesn't have to work so hard - lower CL & Cd.
How can this be true if the stab is at a positive effective AOA? I was always taught the stab is at a negative AOA (effectively, allowing for the wing downwash). This implies the CG is forward of the wing CP to balance the bird.
This creates speed stability. A positive speed disturbance means both the wing lift (up) and tail lift (down) are increased, causing a nose-up moment to restore the trimmed speed.
Or do I need a revised aero text?