Originally posted by Adam
Now this is just weird. I read the posts in the link given and the second one says that AF will not work with this combo -- that the TC will mount on this lens, but it will be MF only. The OP distinctly asked if AF will still work. . .
My first guess (I don't own the Sigma 400/5.6 APO Macro) would be that it might gain an AF lock in the brightest sunlight conditions with a good contrast border, but it would struggle with anything less. Add 1 stop to the f5.6 max aperture and you get an effective max aperture of f8.
I use TCs more than not, and my experience says that an f8 max effective Av combo has little chance of anything close to acceptable AF performance.
Scott