From what I can tell the problem lies with the AMD cards in the new nMP they are only set to support a max of 3840px wide at 60hz (on the nMP) on the 5 officially Apple recommended 4K monitors. Other models such as the Samsung and the LG have various issues that occur depending on the OS version and whether you have the 300, 500 or the 700 version.
I guess the fact the LG is SST rather than MST means it won't go to 60hz either even with display port?
How wide are the black bars, I'm guessing about an inch either side?
Maybe when more 17:9 screens come out support will be sorted, until then sadly I might have to plump for the Dell instead :-(
Yes, black bars are about an inch either side, perhaps a little less.
The monitor will do 60Hz over Display Port. I also have it connected to a Windows box with a Geforce 780 in it which is driving it at full resolution, 60Hz with no issues over DP.