Just curious -- Do you have the 100-400 II + 1.4x III? Four years ago, I bought the 300 2.8 II because the IQ of the 100-400 v1 wasn't good enough. After acquiring the 100-400 II, the 300 2.8 II rarely came out of the house. I found that the 100-400 II + 1.4x III was close enough to the 300 2.8 II + 2x III that I ended up not using the 300. Thus I traded in that lens towards a 1DX2. I'm really happy with the 1DX2 + 100-400 II (with or without the 1.4x III). I also have a 5D4, but haven't used the 100-400 II much with it. The 5D4 is mostly attached to wider angle lenses.