Today I've had the oppertunity to try the new 200-400/4 IS with the 1.4x converter
I've never tried any longer lens than the 70-200/2.8 II IS, so it was quite amazing to have the oppertunity to try this one out. The place had quite bad lightning aswell so I had to use ISO 12.800 on the 5d3, but the images looked very nice.
I really like the fact that I could switch on the 1.4 so easily and the lens felt very "light" (as far as I could tell).. but very well built!
I've attached a picture of me trying it out with the 5d3 here. Have some more pictures on my phone. Have to bring my own 5d3 tomorrow to try it some more!