Re: 2GB GPU to drive a 5k display? @dogged
That would be the issue raised by someone who titled their post "Radeon 290X to drive a 5k display?"
As to heat though, I'll bet Apple has just underclocked it. The advertised fill rate for that GPU is 64 gigapixels per second; enough to fill every pixel on screen more than 4,340 times a second. Let's suppose they underclocked it by 50%, which they won't have, that's naively enough to paint every pixel 36 times at 60 frames per second. Or more than enough to paint every pixel once even when you factor in that advertised fill rates aren't really achievable and blur translucency costs quite a few samples*. And OS X does not repaint every pixel for every frame, it just reacts to the changes (yes, with some redundancy because it's a broad-phase test).
* but not as many as you think; per empirical investigation Apple is using a smart mix of mip map samples very closely to approximate a Gaussian in four or five samples.