how much of Nikon's read noise benefit in the DxO data can be contributed to the zero bias offset (clipping the lower half of the readnoise) of the Nikon Raws?
None
Are you sure?
Yes, absolutely; but I can see that more explanation is required.
First, I cannot speak to DxOMark's numbers; but in the case of their Landscape DR it lines up pretty well with my Photographic Dynamic Range (PDR).
I haven't examined their Engineering DR numbers that closely; Engineering DR has almost zero appeal to me.
The fact that Nikon zeroes their ADU (DN) data makes it harder to determine read noise by the usual methods.
I don't know what method DxOMark uses, but no sensible researcher uses the zero truncated result as the read noise.
So Nikon read noise numbers, and by extension Engineering DR, are not biased (pun intended) by the zero offset.
Furthermore, and wandering close to off topic; Nikon read noise for most models is easier to determine than Canon or most other cameras (!)
That's because for most models Nikon stores unzeroed information in the raw data outside of the effective area (the "optical black" area)
Even if Nikon read noise were made to appear too low by careless measurement; this would have no effect on DxOMark Landscape DR or my PDR.
Those are measured far enough away from zero that the zero truncation has no effect.
PDR is measured using a "wedge" to examine the low end of the Photon Transfer Curve (PTC).
Even excellent cameras such as the Nikon D3S are being examined in the area of 30 to 60 ADU (DN); far far away from 0.
Regards,
Bill