Canon 600mm version 1 vs version II

I'm having 1%'er issues (that's a joke ) trying to decide if I want to upgrade the old 600mm to the new 600mm. Seems any thread along these lines is over 3 years old and I figured it would be good to see what folks think today. Anyone care to chime in?

From what I understand, weight is the biggest improvement. I have also heard that it pairs better with Canon's MKIII extenders.

The Mk2 is a better and lighter lens that focuses faster with extenders. However the Mk1 is still a fine lens by any standards.
Whether the difference in quality and weight is worth the extra is your decision!
I had a similar dilemma back in 2013 and couldn't justify the cost increase to go to the 600 Mk2. However I found a reasonably priced Canon 800 F5.6 L IS. Sharper and lighter than the 600 Mk1 and less need for extenders too. It is heavier than the 600 Mk2 so it sits roughly in the middle for weight. It is also an F5.6 but this really doesn't matter to me as it is sharp wide open and I rarely (if ever) shot my 600 wide open.
If money is no problem then the Canon 600 Mk2 is the best choice in my opinion, but a second hand Canon 800 F5.6 L IS is a significant upgrade compared to the 600 Mk1 and cheaper than the 600 Mk2. It is all down to your personal needs and uses but the 800 works extremely well for me - might do for you?

The 800mm is very tempting especially with the prices it pops up for on Fred Miranda. What worries me about it is the dawn and dusk hours I'm typically shooting at. When trying to maintain shutter speeds faster than 1/800th of a second I'm hitting f4 quite a bit. This especially happens on cloudy mornings.

With that said another 200mm of reach is amazing.

What I'm mostly looking for is better image quality if that's possible. You feel the 800mm is better than the 600mmV1? Has anyone noticed an IQ upgrade amongst the 600mm's?

Poindexter wrote in post #17929009
The 800mm is very tempting especially with the prices it pops up for on Fred Miranda. What worries me about it is the dawn and dusk hours I'm typically shooting at. When trying to maintain shutter speeds faster than 1/800th of a second I'm hitting f4 quite a bit. This especially happens on cloudy mornings.

With that said another 200mm of reach is amazing.

What I'm mostly looking for is better image quality if that's possible. You feel the 800mm is better than the 600mmV1? Has anyone noticed an IQ upgrade amongst the 600mm's?

The IQ on my Canon 800mm is definitely a step up from what my Canon 600 F4 L IS (Mk1) produced. The 600 IS Mk1 is excellent but the 800 is significantly sharper. The loss of one stop can be significant - but it depends what you are doing. I shoot a lot of close range birds (mainly on a 1DX) and am often shooting at F11 to get some depth of field. At longer ranges I am at F8 - it's not often that I use F5.6 even though it is sharp wide open.
As to shutter speeds I find that it is mainly my subject that determines what I need. I can get a very good (hand held) hit rate at 1/500 sec without IS but I often get subject blur so I have to go significantly higher whenever possible.
Note I do not use IS on this (or any) lens as I find that the AF locks and track better without it, same with my 300 F2.8 L IS and previous 300 F4 L IS and 600 F4 L IS.
I had a quick look at your Gear list and see that you don't have any ISO monsters in your bag. In which case an F4 lens will have advantages over the 800 F5.6. I do use my 800 with my 7D2 as well and it gives VERY tight framing (which I like) but it is simply murder to get birds in flight in the viewfinder!
I suggested that you look at the 800 simply because they are reasonable second hand and it is simply a superb lens - very close to the Canon Mk2 superteles and you use extenders less.However if low light performance is critical then it may not be the best choice for you - it was for me but my needs may well be very different.

Thanks for the time spent and suggestions John!

Logic dictates the IQ of the newer 800mm would be a bit better than the 600mm v1. With the maker and breaker of good action photography being shutter speed it is tough to give up that extra stop. I do pair the 600mm with a 1.4tc often and the 7DII is no slouch in the ISO department, so it isn't like the 800mm is totally infeasible.

I'm thinking about it.

In the meantime I'd love to hear if anyone has found a decent jump in image quality when moving from the 600mm v1 to the 600mm II.

I have been told that the IQ jump is significant but unfortunately the only time that I have used the 600 Mk2 was indoors at a camera show so not really going to tell us anything about the real world IQ. I did notice that the AF was quicker though.
I hope someone else can help you here.