DCI-P3 on Mac

It's not really about sRGB vs DCI-P3. It's about color management vs no color management.

One could ask why Apple decided to push a standard for digital cinema projectors - especially since the practical implications are the same as for traditional wide gamut displays: they are simply not usable without full end-to-end color management. As long as you have that, the exact shape of the display color space is of less importance.

Still, it may turn out to be a good thing, because it will force a wider implementation of color management. It has to. Buyers of traditional wide gamut displays generally know what they're getting into. They have made a conscious choice, and configure their systems accordingly. They know they have to use a color managed web browser with these units.

An iMac buyer is a different kettle of fish. They haven't made this choice, and they expect the thing to "just work".

It's important to keep in mind that there are still millions of images out on the web without an embedded icc profile. There are still websites out there that strip the profile if present. The Photoshop default is still (in the new Export module) to not embed the profile. Most web browsers still treat untagged material in the simplest possible way: they just send the data straight through to the display, without any attempt at color managing for the display. That does not work for wide gamut, and it does not work for DCI-P3.

One more thing: I get the impression that the OP treats DCI-P3 as a document color space. It is most certainly not that - it is a display color space. In time it could well develop into a standard color space along with sRGB, Adobe RGB and ProPhoto RGB, but we're not there yet. Older versions of Photoshop doesn't even have this profile installed, unless it's installed separately by the user.

Try it with an image like this: http://www.digitaldog.net/files/2014PrinterTestFileFlat.tif.zip