Don't bother getting the 300+teleconverter, it only degrades the image quality. I don't know about the 200-400, but I own the Sigma 120-400 OS (which is their VR). It has great quality and is around 4 pounds, think. So it's lighter and also much cheaper than the Nikkor.
I easily handhold it and the shots are crisp to around 1/40 second at 400mm.
Good luck.
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