Tom. I tried to duplicate what you get with my D5100. I don't have either of your lenses so the closest I could get with a f5.6 zoom at the long end was with my AFS VR 18-200mm lens. When I lose focus with that lens it stops searching at infinity and not the close-up range you're seeing. I use AF-S and center focus point in aperture priority. Maybe somebody that has either of your lenses can give it a try.
To answer your second question about the performance of higher prices cameras I can only say maybe. I don't know if they are more "intelligent" but they do have more improved auto focus systems in the number of cross type sensors and maybe better response at the f5.6 range.