Hello, I’m the guy who worked on the Nishita sky implementation, as far as I can tell the model is correctly implemented and using the color management properly (of course if somebody can prove it’s wrongly implemented then please let me know).
As a side note, but I think it’s highly related, the default clamp value in Cycles is set to 10, which produces these differences for the Nishita sky Default Clamp.
Here https://developer.blender.org/D10085 is the patch to fix the default clamp value, I think this should be more discussed as it actually makes a huge difference in the final results.
Also, @Eary could it be that the sky shows a higher difference than the default cube scene just because the light intensity is orders of magnitude higher than the default lamp and so exaggerating what isn’t quite visible in the cube scene?