Just to repeat that 1, the Nikon teleconverters are not suitable for this lens (have a read of the lens's manual) and 2, it's not really worth using a third party one since (IMO) I get better results (sharper, more contrast etc) by cropping images shot at 300mm (up to A4 prints). If you want/need the extra 1.7x, the Sigma 150-500 OS is probably the lens for you.