Real world difference:

(Shot using compressed Raw files, so spatial Raw compression artefacts will be visible in places)

When comparing against the Nikon D810, the differences between the two cameras are subtle but definitely present. Even in this challenging, high dynamic range scene, there was little, visible difference in the fully exposed-to-the-right (ETTR) shots. To be fair, though, it's rare to shoot perfectly ETTR Raws, especially given the set of JPEG-only tools manufacturers provide for exposure assessment. To illustrate an obvious difference in shadow noise performance between the a7R II and D810 meant choosing a pair of images that had received 2EV less exposure than a perfectly ETTR shot. Though the difference is certainly noticeable, you're only likely to see it in very high contrast scenes, or after severe post-processing, where the lower relative shot noise in the D810's files - which can receive nearly 2/3 EV as much exposure thanks to its lower base ISO of 64 - means tones are simply more malleable before exhibiting visible noise.

The difference between the Sony and the Canon EOS 5DS is much easier to illustrate. These shots were taken in slightly lower light, so could be given more exposure before clipping and hence are less noisy. Again these examples are 2EV below fully exposed-to-the-right examples, to make the differences more apparent. In this relatively challenging condition, the > 1.5 EV gap in dynamic range between the Sony and the Canon is easily visible. If you wish for the cleanest, most malleable files south of Medium Format when shooting in bright daylight or contrasty scenes, sure a D810 is still king; however, the a7R II represents a fine balance between both high contrast and low light shooting, outperforming both the D810 and 5DS R in the latter (at high ISO). 

Shooting ISO Invariant using S-Log 2

On most cameras it's difficult to take advantage of their ISO Invariant nature because the camera's meter considers you to be hugely under-exposing and, even if your camera will present a preview that isn't related to exposure, the files you shoot are likely to appear almost totally black, making it very difficult to assess exposure.

Sony's S-Log 2 gamma curve: primarily designed for movie shooting, it ends up doubling as a handy way of making it easier to make use of the sensor's capabilities. The ways it does this are two-fold: firstly, because it offers an incredibly flat tone curve, the dark tones in the Raw file that you might want to make use of are pulled up and presented reasonably brightly.

Secondly, though, is the knock-on effect of this tone curve. Because ISO ratings are based on mid tones in JPEGs and the flat tone curve pulls its mid tones from a long way down the Raw file, the S-Log2 mode is considered to be a higher ISO than the normal tone curve. As a result, the lowest available ISO in S-Log2 mode is 800: not because the lower settings are being locked off but because an ISO 800 exposure is enough for a middle-grey object to be represented as a middle grey in the S-Log 2 JPEGs.

Looked at from a Raw perspective and what's happening is the camera is trying to use ISO 800 exposures but with much lower amplification than in standard mode, meaning highlights aren't being amplified out of the file. (If shot with the same exposure values, the ISO 800 Raw files from S-Log2 shooting appear to be somewhere between the ISO 125 and ISO 160 results in standard mode).

As a result, shooting in SLog-2 mode not only provides a more usable preview and output JPEG, it also essentially causes the camera to reduce exposure by 2.5EV to protect highlights.