Kenko 1.4x + Sigma 105mm F/2.8 ...workable?

Does anyone know if the above combo is workable. I know this TC is supposed to work well with lenses having a max aperture of F/4 or greater, but at 1:1 the Sigma 105mm is F/5.6.

If I can use the TC then my lens becomes 147mm. Does this mean the minimum working distance will increase? Does anyone use this combo?

I am also interested to know if AF will work using the Keno 1.4x with the 70-300 VR. Anyone tried this?

Cheers, Alex