After some discussions with Fat Dave and other members and lots of experiments and measures, I am using a tweak to the iris service menu that works very well for my setup.
First a note of caution/disclaimer: Mucking with the service menu in the Sony is not something you should do unless you know what you are doing and are super careful. Also you should write down all of your settings prior to changing anything because many settings are machine specific and if you want to go back to the default for something you will not be able to ask another member and reuse their values. Also it is not known whether making these changes can physically damage your unit so make any such changes at your own risk.
I took my starting Iris Closed Reg and reduced it by 100, and left the Iris Open Reg setting as-is. This has lowered my black floor at 0 IRE and fade to black by 50% compared to the default iris settings. As a result my on/off CR has gone from about 80,000:1 to 170,000:1.
Before getting overly excited about this very high CR, keep in mind that this sounds better than you would think in practice, because much of the black floor is lost as soon as there is an object that requires a decent amount of light (black floor blooms). IOW fade to black is very inky but then step from there one notch up involves a noticeable and fairly significant raise across the entire black floor.
With my previous experiments I also raised Iris Open reg. IMO this is what caused the brighter APL scenes to be rather flat looking. This time around I just reduced the Iris Close reg. This results in the iris being much more aggressive and closing down further on low APL scenes, while not impacting the gamma applied by the DI algorithm or changing how the iris works in high APL scenes.
The downside to this approach is that the iris now has a much greater range of operation to cover. This results in slight pumping, meaning that within the same scene you may see the scene get a little brighter or a little darker. In some cases you may even see it go back and forth, as it gets a little brighter, then a little dimmer, than brighter again and so forth, and can possibly go back and forth like this several times.
If you find that occurs too often for your tastes or is too noticeable/distracting, it is possible to reduce this side effect by switching the iris from Fast to Recommend. And you can reduce it even more by going to Slow from Recommend. I prefer Fast since it makes fade to black very striking, but Recommend is not a bad compromise.
Another side effect is that when you go from a low APL scene to high APL scene (such as bringing up the TV guide for example) you may notice the high APL scene fade in very quickly (within 0.5 seconds) rather than just being at full brightness instantly. This does not bother me.
And of course another option would be to not reduce Iris Reg by 100 but by a smaller amount, such as 70 or 50. Just keep in mind that the less the reduction the less then benefit of the iris tweak.
Now what is also very interesting about this tweak is that it has enabled me to run with High lamp mode and still be satisfied with the black floor. Within the iris trick I did not like High lamp (except for 3D) because fade to black and low APL scenes high too high a black floor. But with the iris tweak the black floor on high is still darker than the black floor without the iris tweak on low.
As a result I am now running high lamp with the iris tweak as my go-to setting. This also results in an on/off of nearly 200,000:1 (well, actually 199,700:1 but who's counting
). This also helps offset the reduced brightness levels in low APL scenes as a result of the iris being closed more. And although the black floor and black level in low APL scenes is darker yet with low lamp and the iris trick compared to high lamp with the iris trick (35% higher) it is not really that noticeable of a difference.
Having now found a nice iris tweak and having calibrated in 3D, I think I'm not fully done with tweaking and experimenting. Kinda of glad to have that wrapped up, but now a little boring too.