I have been asking the same question about a Nikon 1.4 T.C.on a Sigma 120-400 lens. They couldn,t say yeah or neigh at Jacobs in Leicester and suggested I bring in the lens and they would try the T.C. on it. When I got home I found the info that came with the lens and the the way I understand it is that I would need to buy the Sigma T.C. to use on the lens.
I really would like to stick with Nikon equipment though.