If the frame buffer is 8-bit and in P3 color space, doesn't that mean that there are (way) less than 8 effective bits per channel for sRGB content? That seems like a pretty big compromise if true.
The P3 iMacs are 10-bit, and there the color space looks more like a clear win.
That's correct. There is unfortunately banding since it's only 8-bit. It is definitely an unfortunate short term compromise, though the P3 gamut is still worth the tradeoff.
Also, both the 5K iMacs are 10-bit, while the 4K iMac is actually 8-bit. It's down to the bandwidth of the GPUs.
The 5k iMacs use a 10-bit interface, but the panel is still 8-bit and dithered.
Personally, I am extremely happy with the 5k iMac panel, color-wise (contrast is not great though), dithering seems to work well and it's imperceptible in normal conditions.
Nah, it's both 10-bit and dithered, promise. See the Apple reference documentation I just posted above.
You can tell by comparing a test image across the 4K and 5K iMacs. I first discovered this by doing that comparison at an Apple Store. There's obvious banding on the 4K model that isn't there on the 5K model.
The P3 iMacs are 10-bit, and there the color space looks more like a clear win.