"I'm sorry; not seeing any color noise, posterization, and banding. Not really seeing much of a noise cost at all to shooting ISO 100."
I think we should all be able to agree that there is also no color noise, posterization, or banding in my "real world" 100% crop as well, though that does only have an approximately three+ stop lift.
To see the banding you have to lift shadows ridiculous amounts, 6+ stops. The posterization though can be seen in histograms relatively early on though but only really becomes a problem when, again, making silly shadow lifts. The color noise surprises me though, as this is something I see when lifting ~4 stops or so. Maybe an A7 vs A7R thing.
Ok, keep in mind that at 6+ stops you need to expect noise levels of ISO 6400 and above. C'mon. Let's be reasonable.
And yes, more than 5 stops of pushing and you might see some effects of downstream read noise or quantization error, compared to the ISO 6400 or higher (not pushed) shot.
And, yes, different sensors will perform slightly differently here. I believe the A7 is a slightly older sensor, but it shouldn't be accounting for the drastic differences in your experience vs. what I just showed above.
A 6+ stop push is where I'm most interested because most of my shooting is above ISO 3200. Once you get to this point the midtones look noticeably worse for an ISO 100 shot pushed up than a native high ISO shot; this is the issue I have with the claim that you keep all the dynamic range. You can keep the highlights but the shadows and midtones turn to garbage. The reason I mentioned the histogram is because while you can't really see the posterization at ISO 1600 in the image, you can see it in the histogram already and it becomes visible in the pictures once you start trying this same technique at even higher ISO values.