I seem to have the same issues as above, regarding stutters and lag. Using SwitchResX I disabled the internal monitor, and still perceived consistent stutters - I do not know whether your linked script, @Dschijn, does it differently. It was using a 980 ti and a cheap-o PCIe riser - though I have to say, performance was buttery smooth in Windows 10 with no perceived lag and way over 60fps in GTA5 4k at near-maxed settings - again, without any lag.
Any ideas? Currently considering going full into the e-GPU platform with 3x4k monitors, and obviously I won't be able to use those with my MacBook Pro 13" Retina from Late 2012 without and e-GPU - I'm using the AkTiO Thunder 2 case, by the way with an external PSU.