I just "wasted" the most part of this weekend testing all different combinations of resolutions with switchresx, I learned a few things about how screens work etc...
I find 1080p on 27.5" is wasted space and non hidpi 1440p is wasted money.
So I want 1440p hidpi and to get it I have to scale to 5120 x 2880, but this is always rejected. (this is exactly the resolution of the 5k iMac)
I was however able to set a 2880 x 5120 scaled resolution (the same, only rotated). This made a rotated hidpi 1440p setting available, resulting in narrow image at the middle of the screen with supersharp UI etc.
This seems to be proof that the little MacBooks can deliver all the pixels for a retina 5K Desktop environment and fit them on a 4k display at ~52Hz via DP.
The problem is inverting rows and columns.
Messing around I was able to even scale up to 3834 x 6816 with analogous results...
Also when I rotate 90° from standard OSX system prefs I get a retina 1152 x 2048 default resolution which is still more than a rotated 1080p and I get the "pretty" UI scaling menu.
There seems to be much more freedom in setting 9:16 aspect ratio resolutions than the more obvious 16:9, this makes no sense at all, maybe it's some sort of marketing scheme to make me buy an iMac.
I wish this feature was unlocked soon or otherwise hacked out.
Hope this info is useful to someone, I'll just keep on working on my huge 1080p UI now...
System:
Late 2013 256GB 13" rMPB (ie. 1536MB Iris)
Samsung U28E590
