ski.the.east3200 looked questionable in tests I've seen but thats simply viewing online videos that have been compressed further, I haven't used it personally. But I have shot with a GH4 a few times and the image fell apart at ISO 800+ so any improvement over that is much needed
I guess it comes down to what you're shooting, if its skiing in daylight then 2x crop doesn't matter at all, in fact it helps keep more shit in focus for followcamming with tighter lenses which is nice. There are many situations outside of skiing that benefit big from having a clean image with a higher ISO and thats where having APS-C/35mm helps. 4k/60p and no recording limit is so damn nice though, I hope those stats get Sony's attention
My gh3 struggles above 800, which has only limited me once in my current line of work, but you're not wrong, if you need to shoot in low light situations, the gh series isn't ideal, but the gh5 is getting better.
the difference between a full frame and m43 is huge, obviously, but I've used cameras with much, much smaller sensors. Everyone is spoiled by their full frame, which IMO is too big for filming anything, but it works. The tokina 11-16 is still wide as fuck at 11 on a 2x crop sensor and as you said you get more reach which is great too. The m43 is a great skiing sensor, dont let anyone tell you otherwise ;) althought i'm sure you agree with me, thats just an overall statement.
The a7s is a great camera, especially the mk2 but its also $3k and the screen sucks, the batteries are weak, every camera has cons.