I Have a Kenko x2 300 pro. It will not focus on my Nikon D3 With the Nikon 80-200 F2.8 or the Sigma 150-500 OS HSM. The focus just hunts back and forth.
Found this very useful table on Nikon Cafe -
http://blog.nikonians.org/archives/N...er%20Table.pdf
Should give you an idea of what x2's can or can't do.