Having used the Sigma 50-500 "Bigma" and getting some good shots, i found using the Nikon 300mm AFS F4 Plus the TCE II 1.4 Converter a better choice.The images are a tad sharper in my opinion. I know you don`t get the reach but i found getting that bit closer to my subject paid dividends.Have attached an example of what the 300mm /converter can do.
Jon