I've just had a play adding the 2x Sigma TC on the D700 and 70-200/2.8 VR. Interestingly, it kind of works and kind of doesn't. Observations: camera doesn't report correct max aperture - still says 2.8 when it should be 5.6 AF works but hunts. It'll focus to the correct point, but instead of stopping there, constantly "vibrates" around the correct focus point. I can AF so it gets it approximately right then manual focus, but it's getting a bit of a handful when handheld. So any tricks / hacks to get it working, or is the easy answer just buy a Nikon 2.0 TC ?