The Kenko 1.4x MC4 DGX is not fully pinned, and effectively behaves like a taped Canon 1.4x, so it seems to work pretty well on all bodies.
I have the 5D3 and Kenko 1.4x MC4 DGX teleconverter. It works well in moderate to good light with the 5D3 but there is some moderate vignetting at extreme corners. The price we pay for a 140-560 mm lens, I suppose.
It also works well on the 5D3 when stacked with the Canon 2X Mk III (or Mk II) on the 300/2.8 - better than stacking the Canon 1.4 and 2X which focus very erratically. It seems that the Kenko slows down AF a bit and avoids the rapid back/forth focus search that occurs with the Canons stacked.
It seems that the 7D's AF system is not as sensitive, and I had trouble with any f/8 combination in less than perfect light and a high contrast subject. It was very erratic, and would not obtain AF lock most of the time.
One word of warning - don't try to set up microadjust with the teleconver & 100-400 combination - I did this and the camera started locking up. Had to pull the battery to reset it and release the shutter. When I removed the microadjust for that combination, it worked fine again.