I put a crow bar into one of my savings accounts and purchased a Kipon Mount Adapater, so I could use the most excellent Canon L glass with the Olympus EM1 body.
I did notice that it is slower to focus than it would be on the Canon Body. I expected that, the same things happens when you use the Olympus 4/3 adapter on the MFT body. What I didn’t expect that unlike when I use the 1.4x L adapter with the 100-400 mkII L, with the Kipon, there was no difference in the focus speed. IOWs it wasn’t any slower than using just the Kipon adapter. And unlike when you use the 7D2 with the 1.4x adapter, you don’t lose all the focus points, being ‘stuck’ with the center one ony.
What really surprised me is you don’t lose the autofocus when using the Canon 2xmkIII adapter, although it does become a bit ‘nose heavy’. I would suggest using a monopod when using the 100-400 with the 2x extender.
Image quality: So far so good! I took many photos of the bees in the blossoming agave. The images are just as good, if not better, than when I used the Lumix 100-300 (Sorry, Panasonic), but I am using L Glass.
Zooming in, clear sharp image.