AMD (they make the GPUs in the current Macbook Pro 15") have confirmed they will be shipping mobile GPUs next year with display port 1.3. This means you could run a 5K monitor off a macbook pro through a single cable. I think we will see the Apple 5K external display this year.
http://www.anandtech.com/show/9836/amd-unveils-2016-vistech-roadmap
http://www.anandtech.com/show/9836/amd-unveils-2016-vistech-roadmap