The weight difference is substantial, and much better balanced, world of difference when you're walking around with it all day. I've owned the version 2 so I can attest to what I say here. The "reported" better IQ is just silly, if I put images from both side by side you wouldn't tell the difference. Majority of people reporting this silly fact most likely have never owned these lenses. This is just people trying to get too scientific, and it's stupidity if you ask me. I wouldn't pay this much if it rendered a soft image. It doesn't.
Funny how you have no problem with image degradation when you request a built in extender. Cause regardless how little, there still will be image degradation.
Certainly the weight difference is significant, and it would matter to me if I had trouble carrying the 600/4 II. But I go on hikes of a few miles with it and shoot handheld, so... I do use a monopod with it if I'm going to be standing in one spot for a few minutes, and I use a tripod/gimbal for raptors in winter, when I'm in the same spot for a few hours.
The bare lenses are not really different in terms of IQ (the MTFs from Canon do show that the MkII is very slightly sharper as borne out by anecdotal information, but they're really close, i.e. 0.90 in the center for the MkIII and 0.91 for the MkII). No doubt comparing them side by side no differences would be evident. So, I expect that I'd see no real difference between the MkII and RF lenses in terms of IQ...for about 23% of my shots.
I want the built-in extender because about 65% of my shots with the 600/4 are with the 1.4x, and another 12% are with the 2x. That's the use case where the difference between the MkII and MkIII/RF becomes evident, for example see the
comparison by Bryan/TDP – that's a difference that will show up in pictures. Bryan tested two copies of the MkIII, the bare lenses have similar performance, one copy is a little better with the 1.4x and the other is a little better with the 2x but both are meaningfully worse than the MkII with either TC. The RF 600/4 is similar. So for the majority of
my shooting with a 600/4, the MkII will give noticeably better output than the MkIII/RF.
Next logical question is why not get the 800/5.6 if I shoot mainly at 840/5.6 anyway? When the 600/4 II came out, with the 1.4x it was optically better than the EF 800/5.6, as well delivering a longer focal length and costing less. The RF 800/5.6 is really just the EF 400/2.8 III aka RF 400/2.8 with a purpose-built 2x TC behind it, and so it's not a surprise that the EF 600/4 II with the 1.4xIII outperforms the RF 800/5.6 just as it does the RF 600/4 + 1.4x.
So I suppose I should modify my comment that I'd swap my EF 600/4 II for an RF 600/4 + 1.4x in a heartbeat. I'd do so only after seeing the optical performance of the lens. I'd probably preorder such a lens anyway, but make the decision on keeping it later based on optical performance compared to my current setup.