RE: High wing vs low wing
Another element is the drag associated with the landing gear being in close proximity to the wing on a low wing. If the LG is mounted to the bottom of the wing, it will inherently cause more drag than the way the LG is mounted on most high wing Q500's today, with the gear struts extending from the corners of the fuselage.
It would be hard to measure this difference without a windtunnel, but since we're talking theory, the 'gear factor' should make the high wing design better than the low wing.