If I read it right, your P45+ which does have hardware gain to ISO 800, should be better for noise at higher ISOs than backs that don't have it, at the expense of reducing DR at the highlight end.
IMO, this is better than getting full DR but with greater noise in the shadows.
I suspect that was the long thread over there and it was panopeeper that made that comment. What I can tell you is that my P45+ does have hardware gain while Guy's P25+ has software gain. Different pixel pitch, so not a fair comparison maybe, but at the end of the day, BOTH backs behave so similarly with respect to higher ISO as to be identical... So I for one do not buy that first assumption.
As for DR, you always lose some at ISO's higher than native, regardless of how you get there, because amount of noise is a base variable for determining the lower limit of DR -- the more noise, the less DR. Since ISO's greater than native almost always mean more noise, they will also almost always mean less DR.
Note that in all the tests Guy and I have done together, the P25+ does appear to have a bit lower DR than the P45+ at higher ISOs -- like maybe 1/3rd stop. But it also has better interpixel contrast (clarity), and if you add to the clarity slider to get the P45+ file looking like the P25+ file, all of a sudden DR is virtually identical.
In summary, I am not arguing the veracity of the hardware versus software ISO/DR claim, I am only saying I remain unconvinced it matters in actual use...