Lenses are usually lighter if the maximum aperture is narrower, so one would expect the extra weight due to the longer focal length to be offset by a 1 stop slower lens, compared to the 600/4.
It's a pity the 40D does not have auto-focussing capability at f8. This 800mm lens plus 1.4x extender would give an effective reach in 35mm terms of 1800mm.