In another thread a comment was made that I was unaware of, you can in certain situations shoot in auto focus with a lens and tc in excess of f8.
1st shot by R MAURINS, on Flickr
This is deliberately shot in poor conditions, through a window in poor light, no photo shop at all.
x2 tc fitted by R MAURINS, on Flickr
Taken from the same position, just to test that auto focussing does take place.