In the last couple of weeks I've had a chance to do some actual running in these, science-style (read: wearing version 5 on one foot an version 6 on the other, alternating which foot is which for each run). I've also put in a decent number of miles in the 6s alone. Here are my updated observations.
Comfort & Fit
I noticed that the 6s felt different on my feet than the 5s, but I didn't form a real opinion about this until I actually ran in them. And yes, the verdict is they ARE demonstrably different in terms of fit. The heel cup definitely feels a bit deeper, and after a while this became a little uncomfortable. Whereas the heel cup on the 5s feels like it wraps right around my heel and holds it without any weird spots, on the 6s, it feels as if there are lumps inside of the heel cup that press up against my heel. It's not horrible; just not super comfortable.
The other definite difference is related to what I mentioned in Part 1 about how the shoe feels more squashed, top to bottom. Just standing/walking, this kind of just felt like my foot was getting a warm hug from the shoe. Running, though, is not quite as comfortable. I had to loosen the laces completely so that they weren't putting any tension on the rest of the upper, and even then, my foot still feels a little squashed. It's particularly bad on the outside edge, near the pinkie toe/outer ball of foot area, and I often get the sensation of my foot sliding forward in the shoe so that my toes are rammed into the end. This gives the sensation of running in a shoe that's too small, except with extra space in the heel.
For me, it's not bad enough that I can't run in them or want to return the shoes. Like I said, all my 5s have gotten kind of loosey-goosey & needed tighter lacing as they got older, so I'm curious to see whether with a little breaking in the upper on the 6s loosens up a bit & starts to feel a little more comfortable. Even so, I suspect this version will never be a marathon/long run shoe for me. (Back to Brooks Launch, I guess.)
Flexibility & Support
This is why you really need to compare two brand-new shoes side-by-side if you're going to say anything definitive about how the two compare. Putting on just the version 6s, my first thought was, "WOW, these are SUPER stiff compared to the 5s!" And compared the ones I've been running in, they are. But then I put on the brand-new 5s & was kind of startled to find that they felt equally stiff. Apparently, ~200 miles is all it takes to mush them out a bit (which maybe explains part of why I don't like wearing them for long runs after that?). So in that regard, I think the flexibility of the two versions is about the same. No detectable difference folding/twisting them up by hand, either. Like every other version of Kinvaras that I've owned, these are more bendy than than the vast majority of traditional running shoes, but they are not by any stretch 'minimalist.' (WHY WON'T PEOPLE STOP CALLING THEM THAT??)
Also, like every other Kinvara ever, they have a decent amount of support for reasonably efficient runners, particular if you happen to be a mid- or forefoot striker. (Probably fine for heel strikes though there are also probably better shoes out there for that.) They are neutral, though, so if you have an overpronation problem, these shoes are not going to fix that.
Not much to say here -- the 6s felt pretty much identical to the 5s in this respect in every way, if you can ignore the differences in fit. (Though I have clearly gotten used to the old ones I've been running in lately & the amount of ground feel you get from mushier shoes. The brand-spankin'-new ones, both 5s and 6s, did not feel quite as good.)
Really, the only issue for me with the Kinvara 6 is the squished-up, pinchy nature of the upper in the heel and outer toe box. Like I said above, it's not a *huge* deal for me, but I don't imagine I will generally run more than about 10 miles in this version. I think whether or not I buy another pair will probably come down to whether the upper loosens up at all as they get broken in, to what extent, and how long it takes.