The Samsung U28D590D doesn't expose the SST/MST choice in its settings, and I'm having trouble finding anything definitive online as to what it supports in that regard. The manual does state:
3840 x 2160 @ 60Hz (DisplayPort)
3840 x 2160 @ 30Hz (HDMI)
but nothing more detailed. I did find an article that says the Samsung does use DP 1.2 with MST. The Mac is reporting 3840 x 2160 @ 60Hz so that is probably correct, and enabled by default.
http://www.nag.co.za/2014/04/01/samsungs-4k-monitor-is-really-cheap/
Other articles I found are unsure on the specs.
It seems to be a bit of a consensus that all of the current UHD is a little too bleeding-edge, with the obvious problems like I'm seeing here. MST to achieve 60Hz is definitely a kludge, it's impressive it works at all. HDMI 2.0 and DP 1.3 will make things a lot simpler; those will emerge first in the monitors, and eventually in the integrated and discrete chipsets. Which means... another upgrade cycle! I'm pretty much OK if I can't get this to work at full UHD with the Lenovo + HD 4400, it was mostly just to test the UHD waters. I'll end up giving this display to one of my Mac users, they can go blind using it at 3840 x 2160 if they want. I will probably wait for 2nd generation UHD and keep buying 2560x1440 monitors for now for users who want that much display.