Honestly, I think the biggest problem, is that you may simply be underexposing to much.
dual_iso does not fix
shot noise. If you need clean shadows, it's not simply a case of increasing ISO to remove electronic noise, you also have to
expose sufficiently also. Exposure is controlled solely with lens diameter, shutter, and aperture.
When you attempt to capture highlights, that you would otherwise simply blow to white, without dual_iso, you're more likely to lower exposure to capture these highlights. Here, you are pushing the midtones/shadows further into shot noise dominance. Note: Generally, this situation would be pushing the exposure into read noise dominance, but since dual_iso lowers the read noise significantly, shot noise becomes more relevant.
If you're underexposing these highlights, that you would otherwise blow to white, then the situation becomes worse. You've underexposed your midtones with dual_iso (to capture the highlights), and then you are underexposing even further.
You almost always have to increase exposure in LR, because you are underexposing your midtones with dual_iso.
Lets me try and show an example. Let's say that this is our scene lighting.
+------+------+
Lets say that the above represents 14 EV of dynamic range. The middle + represents the midtones, and these midtones are 7 EV below the highlights. Remember, normal ISO is rated for around 11 EV of DR, and this is at the noise floor of the camera. The DR capability of the camera, where noise levels are deemed to be significant, is actually closer to 9 EV of DR.
So in normal circumstances, you might expose as such.
+------+---+
In other words, you sacrifice 2-3 stops of highlight detail, to expose your midtones correctly. In raw ADU's, the midtones are around 4 EV from sensor saturation (overexposure).
So where we use dual_iso to capture these highlights (that would otherwise be overexposed), the midtone point has been shifted 2-3 stops lower (instead of being 4 EV below sensor saturation, it is now 6-7 EV below sensor saturation). Hence, in PP, you need to increase exposure 2-3 stops to compensate. This isn't a deficiency with dual_iso, it is an expected outcome. a1ex's soft-film highlight compression can be used to automatically compress these highlights, so that the midtone point of your exposure (the rendered image), is more closely aligned with the midtone point of a standard Canon image.
Here, you might not need to increase the exposure in PP, because soft-film is applying an exposure correction automatically. I will agree though, that in it's current state, soft-film is difficult to use, because it requires manual intervention regarding the burned WB values. a1ex has mentioned that he is interested in using WB processing, to automatically determine the "correct" value for soft-film use.
Remember, raw based processing, is all about the raw processor determining what the 0's and 1's are
supposed to represent. What you have to do in PP is irrelevant. The determining factor, is ensuring that your
exposure is correct,
during capture.
