I ran a simple test today with the above mentioned combo today. I tried as best I could under the time I had to match up magnification, hence why the 200-600 might not be at rounded off focal lengths. I wanted to tripod a test @100 iso as Ive only seen high ISO tests. And I wanted a relatively flat scene. Finally, I shot wide open and stopped down a bit without going deep diffraction, I want to know usability for me.
Its by no means thorough but as Im 2 days post surgery its all I had the energy for today. My 100-400mm I have enjoyed as the better of the recent samples I have tested. The 1.4x is not mine so I dont have any long term views on it. I only have 1 x 200-600mm at the moment so there may be some sample variation with them as they ship next week.
So here you go... I hope it helps.. For me... Im sticking with the 100-400mm GM for now.