Surely an aft CG helps improve cruise performance?
It sure does! Thanks, I made the correction.
Stability is one thing and equilibrium is another.
Negative lift at the tailplane is a requirement for the latter. It is due to the wing aerodinamic moment, which is nose down for positive lift flight. Tailplane provides nose up with negative lift, thus balancing moments.
It also improves stability. An airplane disturbed and put into a dive will pick up speed. As it picks up speed (if the tailplane is producing downforce) the wings will decrease AoA and the tail will increase AoA which is what causes the pitch up to restore the airplane towards it's original path. Elevator trim doesn't trim for airspeed, it trims for angle of attack. And when you disturb the airplane from it's trimmed AoA, it will try to return to that AoA. Since the trim is trying to maintain a certain AoA, the wings will also try to return to that AoA and, in doing so, will produce more lift which will also help bring it back to it's original path.