Nikon says dynamic range will be as good or better than that of the D810, despite the higher pixel count.
They've stated that there is no trade-offs to be made in balancing dynamic range at base ISO vs. higher ISOs, and that this sensor resolution represents the optimum balance for performance and image quality.
12) The Nikon D850 should be a significant step up, in terms of high-ISO noise and image quality.
While they didn't attribute it to the BSI sensor, Nikon told us that the D850 should produce the same image quality (both JPEG and RAW) at twice the ISOs as the D810, a full-stop improvement. That is, the D850 at its top "native" ISO of 25,600 should deliver the same image quality as the D810 did at ISO 12,800. If true, that's a pretty significant improvement; we can't wait to get our hands on a sample to check it out!