excellent and through information. You compared to Eizo and NEC - what are the areas where they are still better?
I'd be surprised if this monitor is Eizo quality. What retail, stand alone monitor is the iMac 5K 27 inch based on? What retail model is the closest?
I have used Eizo ColorEdge series recently, and NEC Spectraview some years ago. They are professionally graded reference monitors for printed media and photography (and recently video as well). Nowadays they use the same LG / Samsung panels as the other brands' products (including Apple's), but they excel at the circuitry, the firmware/software integration, calibration / LUT flexibility, general build quality etc. They also seem to get much more aggressive QC in checking uniformity and IPS light bleed, in the many dozen copies or Eizo I used at work I have probably only seen bleeding in two in a total dark room, and never inconsistent tint / brightness that is discernible by naked eye. I have "heard" that perhaps the 1st grade panels are reserved for vendors like them, while Dell / Apple gets 2nd then the rest get 3rd crop (*citation nonexistent).
Some models also have included a built-in spectrophotometer, which can be programed to self-calibrate on schedule. The driver/software also works more tightly with the OS and has records of calibration errors/consistency for reference. They also of course come with tank like build quality, the older CCFL (tube) ones even had fans for cooling (maintain brightness consistency), and none of the thin-bezel nonsense (which creates un-flat surface and bleeding due to lack of pressure in corners). They always come with a fully 3D adjustable stand, and use matte coating that has minimal viewing angle contrast offset.
Apple branded displays while generally good, thoughtout its history there was hardly a point in time where Apple offered anything at that grade. The current iMac 5K probably is the closest in terms of out of the box quality. But all the other features and specs add up, for color critical workflow you still need one of Eizos or NECs. But I guess for semi-pros or even creative pros earlier up in the production chain, the iMac screen more than suffices.
If I am not wrong, the 5K panel is the same LG since the first 5k iMac (2014?) and the Dell UP2715K. The difference in the LG Ultrafine and the current 2017 iMac is the backlight using better LEDs so we get a higher peak nits, and thus higher contrast ratio.