Not so sure you're correct. It may be a limitation in OSx---but quoted from Notebook Check...
"The Intel HD Graphics 5000 (GT3) is a processor graphics card included in some of the ULV Haswell processors of 2013. It represents the highest configuration of the integrated graphics unit (40 execution units), but lacks the additional eDRAM cache of the Iris Pro Graphics 5200. The relatively low base clock can be automatically overclocked using Turbo Boost technology. Depending on the processor model, the base and turbo clock rates may differ, resulting in varying graphics performance between CPU models.
In comparison to the HD 4000, the graphics core has been modified extensively.
The GPU supports DirectX 11.1, OpenCL 1.2 and OpenGL 4.0. It also features an improved decoder for 4K videos and the fast Quick Sync encoder."
Again---OSx, in it's current (10.8x) may limit external monitor resolution, but with Mavericks--it may allow external 4k output to an external monitor. Thunderbolt has the bandwidth, the '5000' has the grunt---and 1600p, are you determining that by using the 16:9 or 16:10 ratios of the current displays? It's a helluvan un-even number between 1080p and 2160p (4k's vertical resolution). Just curious as to how you came up with that number. We do video production, all in 1080p these days...but as the prices of 4k cameras drop...it'll make sense, soon, to shoot high-rez, even if distribution is 1080p....as we are, or will be (5 years?) provided the opportunity to 'view' 4k content in the home, once they figure out how to distribute and the compression measures used so as to not take up 200+ GB per flick
😉
J