Thanks to AusMacFan contribution, I was able to compare the new P3 iMac display to my reference Dell AdobeRGB wide gamut display. This is strictly a color gamut comparison, ignoring any other variables, like screen reflections, shadow grayscale and gradient responses, etc.
The Dell wide gamut display has stronger blues and cyans, true to its claim of 99% AdobeRGB coverage. The late 2015 27” P3 Retina iMac Display surpasses it only slightly on the magentas and reds, but is considerably larger on the green-yellow-orange parts of the spectrum.
Overall shape is very similar to the DCI P3 color space, falling a little short on the greens and blues, but actually surpassing it a bit on the purple-magenta axis.
Which color gamut one is more useful?
For video, definitely the iMac, since it corresponds closely to a video standard.
For web, both represent a real problem until all browser vendors adhere to the color management guidelines suggested by the W3C. In a nutshell, only Firefox (if properly configured) and Safari do color management right on the Mac platform. Chrome interprets ICC v2 profiles embedded in images, but fails to apply any color management to the other page elements, rendering them on the full display gamut. P3 iMac users will see oversaturated colors on Chrome, as any other wide gamut display users.
Compared to a high end inkjet on fine art baryta paper (Epson 9900 and Epson Exhibition Fiber), the P3 iMac display falls short on the blues. The Dell AdobeRGB wide gamut monitor covers almost all of the printer colors on the blue axis, but, on the other hand, fails to achieve its full gamut on the yellows and oranges.
For photographers, it’s a wash between AdobeRGB wide gamut LCDs and the new Apple P3 displays. Both represent a tangible improvement compared to traditional sRGB displays, specially for those doing fine art printing.
Keep in mind that those gamut limitations don’t necessarily represent a problem in real world usage. Color managed apps map out of gamut colors to the display gamut, keeping the overall image representation plausible and accurate.
Looking for a previous generation 27" 5K iMac ICC profile for comparison.