max 3200x1800 resolution on 4k monitor??

Good thought. I tested it with the three different buses, same result.

Funny enough, just looked at my 'About This Mac':

It shows the Aus as having 6400x3600 resolution, which is not correct.

Here is the screenshot of my Display Prefs:

Both displays are driven from a D700 in slot 2:

Also, when the screen comes back online (after sleeping, etc.) I might have the 3200x1800 resolution back up. I am using a DP 1.2 cable sourced from Amazon.com.

Thanks. But not sure how this is helpful.

No. What is also strange, is that the usual 'Retina' setup does not appear, unless it's on 60 Hz...

here is the shot at 60Hz

Not that I am aware of. Not sure the MP supports HDMI 2.0 either.

Will talk to Apple Support to identify, where it is a OS X issue and where Asus. My suspicion is, that it has to do with the drivers of the D700s.

I finally managed to get the screen going at 30Hz and full 4k resolution. I used a DP to HDMI cable for this. To get 60Hz, I need the mini DP to DP 1.2 cable. But this leads to tearing or artefacts on the right hand side.

Maybe I got the max resolution of 3800 x 1800 @ 30 Hz because I used a DP 1.2 cable with the DP 1.1 setting on the screen side?

Consensus on the web is, that the tearing is a driver related issue, as people who have started up from Windows on the Mac Pro (via Bootcamp) do not have the tearing issue. When I chatted w/ Apple Support, they stated. that 4k was still in its infancy and I should be seeing improvements in the not too distant future.

Thanks to Loner T for coming up with ideas.

LL