@kgp do you still have the LG 5K2K?
I bought one today. Works perfectly in Windows.
To be able to access the BIOS, it just be connected to DP2 (on DP1 I have an Apple TB display). With the LG 5K2K in DP1 everything stays black until windows has finished booting.
In macos it boots on a black screen if it's on however. Doesn't matter if it's set to DP version 1.2 or 1.4.
If I turn it on after macos has finished booting, set in DP1.2 the screen shows as 3840x2160 30Hz max.
If set on DP 1.4, can do 3840x2160@60Hz (bit it really looks awful then); it looks best un 2560x1080 mode which is what to be expected.
If I recall correctly, you used a tool to make it recognised at full resolution. Which one was it?
Edit: Okay, found that pressing the option key while looking at the resolution allows me to change res.
Now the issue is that this screen only looks good when using the 2560x1080 resolution. The 5120x2160 looks just too small.
If I connect the screen via TB and use two DP 1.2 (and the screen is set to DP 1.2) then macOS allows to run a native resolution with scaling. The screen when set to looks like 3360x1417 looks very nice and is very usable.
Anyone knows how you could patch so that when connected over DP 1.4 then macOS scaling is available?
You need to use SwitchResX under macOS and define a custom 5120 x 2160 , 60 Hz resolution. By this the LG 5K2K perfectly works under macOS with native 5120 x 2160 , 60 Hz or any other resolution via DP1.4 and with DP1.4 enabled in the onscreen display menu. If you use 2560x1080 use the HiDPI profile, which will be available if everything is configured properly on your system, which implements native 5120 x 2160 , 60 Hz resolution but 2560x1080, 60Hz UI resolution.
Any yes, I actually replaced my former LG38U99W by the LG 5K2K and I am suing the latter every day.