If there was complete engine failure (and all other things being equal), would a passenger jet traveling at 600 mph when the engines stopped glide farther than a plane traveling at 400 mph when the engines stopped?
Logically, you can argue that, sure, the faster plane has more forward momentum and would travel farther. However, I think there is an optimum speed for the glide ratio, and being too fast or too slow would decrease your glide distance.
Thus, the plane going 600 mph has slow down more to get to the optimum speed than does the plane traveling 400 mph, and less efficient glide during that slow down period would ultimately shorten the overall glide distance for the faster plane.
What say you, oh wise internet?