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.
This maybe a vast over-simplification of your question, barit1 - and I apologise if it is, but the MAIN reason why this is more efficient is that the tailplane lift is contributing to the overall lift component whereas if the tail has to produce a nose up force, it is not. Therefore the wing works less 'hard' therefore less induced drag. As 'mad' says, the wing pitching moment is almost irrelevant in the balance equation as it is acting MORE OR LESS around the cg. Aft movement of cg REDUCES the pitching moment of the tail and therefore REDUCES pitch stability.