For my environment (and moreso tastes) I lower both Iris Open and Iris Close to some extent (a greater change to Iris Close).
I had the same perception that you noted, but my measurements didn't align with what my eyes were telling me. I was just accustomed to the brighter picture, so I was convinced it was flattening out the scene. I noticed this first during my HW30 testing, but when I measured
I found the opposite was actually occurring - there was a GREATER difference in measured brightness between the dark parts and the light parts of a low APL scene as I closed the iris further. I duplicated this same test with the 95ES (though in less detail than my long-winded HW30 test), and got the same results.
If one wishes to still have lots of "pop" in the bright parts of a low APL scene, then one would keep the iris more open, sacrificing the black level to some extent, and conversely if one were more interested in darker blacks and could sacrifice that "pop", then one could choose lower iris values. The beauty of having a dynamic iris and good native contrast is that you can maintain whatever brightness you desire in the high APL scenes and still have the flexibility to monkey about with the low-APL scenes to get the best of both worlds. Provided you don't go to such an extent as the iris becomes intrusive.
I'm currently running Gamma 4 with a +1 to the blacks, which gives me a measured gamma ramp that starts just above 2.1, hits about 2.35 by 30, and keeps going fairly flat. I think this is helping me run a more aggressive iris setting and still be happy with the detail I see in the dark scenes.
Unfortunately, I'm not able to have the gamma curve exactly as I like and I'm definitely not able to perform the fine greyscale adjustments within the menu system itself (The JVC spoiled me in this regard). So I'll be hunting down the serial cable today and breaking out the ImageDirector to see if I can start shaping things more accurately.