Funny you say that as I was at my Canon dealer ONLY yesterday and he was tempting me with a 200-400 as well as a near new 800! Good deal too but I managed to refrain - just.
The 600f/4 II + 1.4X III will give you marginally better IQ @ 840 & f/5.6 when compared to the current V1 800f/5.6, and the 600 is a deal lighter & easier to pack as well.
That may be the reason Canon are dragging the chain on a VII 800f/5.6 when you see that Nikon have had their new VR 800f/5.6 (with 1.2x Converter included) on the Market for some time now.
But you did well to restrain yourself on the 200-400f/4, there are very few situations where I miss my 400f/2.8 II, but then I have the 300f/2.8 II as a filler for low light, without that ?? Maybe tears.
Thank you guys.
This was my reasoning for refraining from both the 200-400 and the 800 5.6:
I shoot with two bodies: 400+1Dx and 70-200 2.8 on 5D3. It's f2.8 all the through and suits my shooting style so 200-400 is redundant.
800 5.6 is simply old and NOT a Version II big white.
I'm more interested in the roumored 7D II esp with my 400 2.8