Some more findings:



1) 0x8880 is black level. Changing it doesn't seem to bring any improvement (DR decreases a little, not sure why).



2) The sweet spot seems to be where the white level is still the same as with the unmodified ISOs. Here it's around 0x320 => the real ISO under 100 might be ISO 80.



3) As you know from the dual ISO PDF, you can override the ISO gains from CMOS[0]. Now check the dynamic range of ISO 100 overriden to 1600, and ISO 1600 overriden to 100 (with tweak disabled). Here, the DR is roughy 0.1 EV higher whenever you select 100 in Canon menu.



This means dual ISO should work better as 100/1600 than as 1600/100. Anyone confirmed this experimentally? (though it's quite unlikely to notice a difference of 0.1 stops)



At ISO 1600, the difference vanishes when enabling the tweak. At ISO 100, the difference is still present, and it gets up to 1 stop when you select 25600 in Canon menu and 100 as override. So, there are some other settings used for these higher ISOs (and finding them may result in some more parameters that can be tweaked).