In addition to Brant's comment, the 600 II + 2xIII beats the 800/5.6 + 1.4xIII. I think the 800 is rather superfluous at this point, and I'd expect a replacement soon (although 'soon' is relative).

Nikon did recently release an 800/5.6 VR that's optically excellent (and priced at $18K). It's interesting that Nikon is now using fluorite elements in their supertele lenses (the 800 and new 400/2.8 have fluorite elements and an FL designation). When only Canon used it, Nikon said fluorite "easily cracks," but now fluorite's "superb anomalous dispersion properties...effectively correct chromatic aberration," and it allows "a more effective lens with less weight," (all of those are quotes from Nikon's lens glossary). I guess Canon knew what they were doing when they first started using fluorite elements in SLR lenses...45 years ago (but in fairness, Nikon has used fluorite elements for many years in their microscope objective lenses, since fluorite transmits UV better than glass, an advantage for fluorescence microscopy).