The members on Nikon Rumours were sure BSI gives a one stop advantage for low-light noise. I am not seeing that.
Well, it depends on the metric used. According to dxomark, the dynamic range and color sensitivity of the D850 are ahead of the D750, provided that the images are resized to equal size (8MP) for comparison.
However, photonstophotos.com measure so-called photographic dynamic range, using a slightly different metric, and in those graphs the D750 seems to be ahead of the D850 at ISO 100-400 whereas at higher ISO the D850 is equal or slightly better than D750, if I am reading correctly.
Both sites present normalized metrics i.e. like the photos would appear printed to the same size.
Now, why is there a discrepancy here? I am not sure there is one; the metrics and methods used are different and so are the results. I am sure the keepers of those sites have a lot to say about it. Dxomark measurements look deeper into the shadows than some other sites and it could be there that the D850 improvement is. Nikon may have used the dxomark results as basis of their advertising a 1-stop improvement at high ISO. I think the metrics may not always agree with what a human observer would say, but then two human observers may also come to different conclusions.
My impression is that I am happy to use the D850 up to a slightly higher ISO e.g. in figure skating photography, but the D5 is clearly better. I am not surprised you'd find the D750 to be a bit better also, but I do wonder if you are looking at the final image at equal scale (same display size, not 100% original pixel view). Anyway, the characteristics that one values the most may not be what the designers optimized (it really depends).
My subjective feeling is that the D850 has shaken some of the high ISO problems of its predecessors (D800/D810) but it is still a camera which is at its best at low to medium ISO.