Outside an iMac 5K enclosure? Where?
http://www.slashgear.com/an-apple-thunderbolt-5k-display-dont-hold-your-breath-17351284/
The Dell 5K monitor is supported by the 2013 nMP and the latest 5K iMac since 10.10.3.
There are people on this forum that have confirmed the 2013 nMP can drive the 5K Dell display (same panel as the 5K iMac) using two cables.
Some displays with resolutions higher than 4K require two DisplayPort cables to connect the display at full resolution. With OS X Yosemite v10.10.3 or later, the Dell UP2715K 27-inch 5K display is supported on the following Mac computers:
Mac Pro (Late 2013)
iMac (Retina 5K, 27-inch, Late 2014)
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Which means there is no existing base of Macs ready to support an external 5k monitor with one cable . Maybe at WWDC we we will see new MBP, Mac Pro with TB3 and support for a 5k ATD.
I do not know of any computer that can support a 5K monitor with one cable.