I've been using Canon 5dii and 70d bodies with EF600/4 lenses for birds (different versions over the past some years) and was never all that impressed with AF accuracy, although with enough multiple frames I did get a few acceptable images to be satisfactory. During that time I didn't feel like I should have to spend the big bucks for the top of the line 1 series bodies, that the cheaper, simpler models should suit me well enough.

Canon finally put the better AF system in a body I'm will to pay for, the 7dii, and I was very surprised at the difference it made in accuracy compared to the 70d (+/- 1/3 dof versus 1 dof) with my EF600/4 IS (II).

I suggest that anyone considering a long fast lens also plan on using a body with the better AF system.

I only photograph birds and only with the 600+1.4x/7dii and 70d and a 100-400L and 70d; I prefer the soft light of overcast skies and often work in the rain, snow, sleet and fog (but not wind). You can see that I have rather demanding conditions and I also expect quality images good enough to make display prints.

I've used 4 or 5 Mark I versions of the 600/4 with a 5dII and I wouldn't say that I have experienced remarkable improvements with the 7dii, in fact I found more variability in resulting images from one sample of lens to another, much more in two cases!

But on the 7dii the ability to select a very small focussing spot is most welcome as my subjects often require picking out the head or eye of a bird when the whole animal only occupies 1/8th of the image area or less at times. With the full-size single square AF box selected, I often can't be sure whether the system is picking up the head or the tail of a bird  and examining the resulting files shows this variability.

On the other hand, the supposed improvement due to the mode 3 setting on the 600/4LIS(II) for moving subjects, much heralded by some reviewers, has been a total disappointment, as has tracking ability with something as simple as an on-coming swimming duck (in mode 3, even allowing for microfocus being slightly off).

Also, on the 7dii I'm disappointed that they didn't include the articulated LCD of the 70d which I find very helpful, along with LiveView, to find birds at extreme high and low angles from my car window (a major reason I bought 2 of them and am keeping one). I've had to buy an angle finder for $250 to do that job with the 7dii and not nearly as well.

Despite everything I've read in reviews, I don't get near-perfect sharpness on 90% of my images, when examined at actual pixels (100% view). It is improved compared to my previous cameras and I guess that's all I can reasonably expect doing birds in Oregon's gloomy winters. I hate the look of flash and would rather miss some good shots than produce lots of ugly ones.