Imaging Resource published some interesting D850 facts they got directly from Japan:
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!
While Nikon contracts with a silicon foundry to actually manufacture the chips, Nikon confirmed that the D850's sensor is entirely their own design, vs. an off-the-shelf unit from a sensor manufacturer.
Nikon says dynamic range will be as good or better than that of the D810, despite the higher pixel count.