With 16 pounds dead weight, ie. 2 8lb. dumb bells, there is no gap. The shoulder straps and the remainder of the bag hug my shoulders and conform to the shape of my back snug against it. (5'9" 18 " torso, size M pack).
"How far below your shoulders is the shoulder harness attached to the main bag?"
Doesn't that depend on how tightly you have adjusted the shoulder straps? When the straps pulled tight, the point where the shoulder straps are attached to the pack hans only a couple of inches below the top of the shoulders.
For instance, if you hang the shoulder straps over your arm when holding the portion of your arm between your hand and elbow parallel to the ground, the pack will just hang slightly down off the curve of your arm as it would when over your shoulder.