Just going to follow up with some more info.
First, thanks to pross for pointing out I had a typo in the 'epro' command above, now fixed! 🙂
The difference between Intel Iris and 'classic' Intel is that 'classic' Intel uses the xf86-video-intel driver (with ebuild of same name), which has classically been the way you get accelerated video with Intel Integrated graphics.
The Iris graphics uses the more generic 'modesetting' driver (no special ebuild for this), and uses a gallium-based driver which is now a part of media-libs/mesa so xf86-video-intel is not used at all.
During testing I found that if you have xf86-video-intel installed, you *won't* get the Iris graphics. xf86-video-intel seems to have priority and will get initialized first. So if you are doing troubleshooting or not using the magic mix-ins, you'll want to make sure that xf86-video-intel is not installed if you want to use Iris. The way to check what driver is being used -- the definitive way -- is to look at /var/log/Xorg.0.log and search for 'modesetting' and 'intel' and determine which if any are initialized. In some circumstances, you may see references to both so you need to look very carefully at the end to see which one is active.
Now, about the mix-ins themselves which handle all the complication for you, so you just set the right one and don't have to tweak everything by hand. The gfxcard-intel (gfxcard-intel-classic) and gfxcard-intel-iris mix-ins do the 'magic' for you -- gfxcard-intel-iris will MASK xf86-video-intel, and enable video_cards_gallium-iris, which along with a dep in xorg-drivers will BLOCK xf86-video-intel from being installed (forcing it to be uninstalled.) Thus, the "magic" that gets your system in the proper state for Iris. The mix-ins live in /var/git/meta-repo/kits/core-gl-kit/profiles/funtoo/mix-ins if you want to look at their exact settings. You will see there also associated USE flags associated with each mix-in that are passed to the mesa ebuild which are different for the classic and Iris configuration.