I finally got a little time (and sunshine) to head out to the local nature center to fiddle around with my 600mm VR lens and try it with the TC-20E III. When you are using long lenses, tripods are absolutely mandatory; hand-holding is not at all feasible (well, maybe if you pump iron like Ah-nold).
I started off with my normal combo for small birds: 600mm + TC-14E. This gives an effective 840mm focal length on my FX Nikon D3s. The TC-14E only costs one stop of light, so my lens behaves as though it were f/5.6 instead of f/4 when it is wide-open. I’ve used this combination several times in the past, and it works really well. I get sharp images and AF performance is still very fast and accurate.