I played with various combinations of TC's and a tube last week for another thread here. They were with a 100L and my 5D MK III. The short answer is that quality suffers with the 1.4X and suffers a lot with the 2X. Autofocus is also lost. Using just a short tube and the 100L works well, and autofocus works well too.
I have tried :1DX, 100L, combined with kenko 12 and 20 and 36 and 2XIII.The only way that I could work with this combo was on a tripod and using Helicon software for the focussing and image stacking.Focussing by hand is next to impossible with such a limited DOF.I will post some images later, comparing the 100L as is and with the combo.Hope this helpsBen