Nope. Pic related.
At the same view size the A7s2 is soft and lacks detail, yet it doesn't really have less noise. Its noise is simply soft like the rest of the image. The color noise in these images disappears with default color NR (dpreview naturally turns this off) leaving the A7s2 at a severe disadvantage to the 1Dx2 and the 5Dsr at ISO 12800.
Typical reviews and online memes suggest the A7s2 is one of the best high ISO cameras. The same reviews and memes say the 5Dsr is "not a high ISO camera." I would rather work with a 5Dsr ISO 12,800 RAW file any day of the week.
As for extreme ISOs, the 1Dx2 and A7s2 show the same behavior through 409,600. The noise level is about the same but the noise is literally sharper on the 1Dx2, like the rest of the image. People tend to have an aversion to sharper noise while pixel peeping but at common view sizes the sharpness and detail stands out more than the noise. And the sharper, higher resolution image has more room for NR if the noise really bothers you. No matter how you slice it, the 1Dx2 is the better high ISO camera for stills. (Video can be another beast entirely depending on how the image is captured and scaled off the sensor.)
Since the introduction of gapless microlenses pixel size has not mattered for high ISO, given the same sensor size and level of technology.
DR should, in theory, be affected by pixel size. But in practice we're not seeing that. Some of the highest pixel density 35mm sensors are also the highest DR sensors, higher than MF offerings. And this has occurred not only with Sony's on-chip ADC sensors (Nikon D8x0 line, and now A7r3), but in the Canon line with off-chip ADC sensors. At introduction the 5Dsr was the highest DR Canon body until Canon's first on-chip ADC sensor bodies.
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