The real competitor of Sony A7R III is Nikon D850. Yes! since in sensor review at DXO the camera scored Full Monty (100/100) and now even in Dynamic range test by DPreview D850 gently out edges Sony A7R III. Look at the table below:
You can see from the table when tested at base ISO, Nikon D850 grows bright that produces maximum 13.78 Pixel Dynamic range when shot using Full resolution of 45.75 MP means it cleanly beats Sony A7R III that ends up scoring 13.63 pixel Dynamic range at its highest resolution of 42.4 MP. Interestingly, as you take more sensitivity range (more than ISO 100) the Dynamic range score of Sony A7R III increases from D850. By the way, real test is at there base ISO where Nikon D850 stands strong!
On the other hand, “Sony claims 15 stops of dynamic range in Sony A7R III, which is more than Sony A7R II”, as Sony stated. Now Photons to Photo gone though a depth test and shows the graph below:
Now look at the graph where you can see that as the sensitivity increases the Dynamic range of A7R III decreases simultaneously and the most surprising part is that it completely lays down on A7R II. Now in our eyes we see no progress or improvement in Sony A7R III dynamic range as Sony claimed.
Finally, we have concluded that whatever Sony claimed doesn’t lay down in practical world fully. However, we would say Sony A7R III has made some countable improvement from the predecessor and some other DSLRs in its range.