Well it's tricky because our eyes do not see the linear effect of brightness or IRIS changes linearly at all. Then the second problem is, how sensitive are users to the IRIS moving, this will vary, some will like a bouncier IRIS than others simply because they find it an ok trade-off for the better blacks. All I can say is, this IRIS is not as good as the Panny/Epson IRIS's, but it is better than some of the other IRIS designs. The fact you can customize it really does mean you can make it nearly invisible for a small increase in black level compensation, or make it really aggressive, or even just change it depending on what you are watching.
Hard to give a hard-opinion on how much you'd dislike the black-levels. The IRIS does appear to act on a different CURVE depending on its range setup, but it's impossible to test that since the EYE does not see the curve linearly (so there will be sudden changes seen sometimes).
I'd also have to watch stuff for a good full week or two with the DI engaged, and right now I'm using FIXED Mode, because only way to engage DI is to move my projector to a less bright spot (retro-reflective screen) or add an ND filter, which I haven't done yet. Hence, Itested the DI for a few hours, and that's what I've found so far.