Ok, I've read a lot of reviews and re-read some old ones and some forums.
I can't belive literally NOONE did a job comparing side-by-side these three direct competing models
- all DLP, FullHD, 3D, similiary priced. But after reading alot I came to some conclusions.
Well It's not really clear which has higher contrast and blacker blacks, HC7800D or HD8300. I took a closer look at Art HC7800D review
Both photos have similiar blacks overal, but Optoma (right) is clearly more overexposed, which means in theory Optoma is blacker (?). The same with night train scene:
I'm still not sure is this correct. There are some tests reporting better contrast numbers for HC7800D
2900 native, 10200 dynamic
2575 native, 8900 dynamic
And I also not sure these numbers are correct, because he measured navite contrast for HD87 lower than HD83 (2444:1), which is doubtful. http://www.audiovideohd.fr/tests/249-Optoma-HD87-4.html
Also I found some posts at http://cinetson.org
where people said that HC7800D is darker and contrastier.
Then I looked back at XV-Z17000 photo from Art review
Sharp photo is obviously even more overexposed than Optoma (right) and has slightly darker black.
It seems Sharp have blacker blacks than both Optoma and Mitsubishi. Considering that XV-Z17000 has second manual iris for achieving higher contrast, it totally makes sense.
I found a confirmation on soundandvisionmag.com :
The best black-level/contrast-ratio combination was achieved in Eco+Quiet mode, with High Contrast and Auto On iris settings. In this mode, black level was 0.00086 and max light output was 14.44 (16,790:1).
If you add in the DynamicBlack auto-iris and use the Cinema 2 setting, black level and light output decrease to 0.0012 ftL and 12.99 ftL, respectively, for a dynamic contrast ratio of 10,825:1.
The auto-iris produced the best contrast ratio at the Auto2 setting, reducing black level in Standard lamp mode to 0.0023 while delivering 22.77 maximum light output for a dynamic contrast ratio of 9,900:1
Also there are side-by-side photos in XV-Z17000 avs thread, where Sharp performs (with both irises) equally to Infocus IN82 in space scenes, or even better. https://www.avsforum.com/t/1274146/sharp-xv-z17000-new-dlp-full-hd-3d-projector/200_100#post_20249090
So, now I think Sharp has the best blacks using auto iris and manual iris.
And I would buy XV-Z17000, but the problem is that all reviewers report Sharp auto-iris is unusable, distracting and annoying.
When I took those pictures, I liked the added black levels, but when watching "King Arthur" last night, it kept turning down the light in a sudden and very obvious way. It's imposible to not notice the iris suddenly going from fully open to closed, during a close up of two persons talking. Eventually I just turned it off, it was way too distracting.
It has only one setting - just "on" or "off". And without dynamic iris Sharp is not better than the others.
So what I've decided. I will not take Sharp or HC7800D.
The new Mitsubishi HC8000D-BL is very tempting because it also uses additional manual iris and should be definetelly better than HD8300 in 2D. I don't know the price (should be less than $3000), and don't know when it will start shipping, maybe next week, maybe in two months.
So I'll wait some time.