[lost in the woods]

Hi Stu
Im a canon user not nikon so i can't comment on lens but i own a sigma 300mm 2.8 and that will auto focus with X2 convertor. most lenses will auto focus down to a maximum apeture of 5.6 this is what you would have with a 2X fitted to your lens after f5.6 lenses tend to hunt more due to lack of light entering lens canon pro bodies will work down to f8 on centre focus point only, you can some times tape pins on teleconvertor to get them to work if appeture is less than F5.6 but don't expect miracles.
Regards
Lost in the Woods