After some testing on a Skylake machine, I've concluded that xf86-video-modesetting is far superior to xf86-video-intel on that generation of Intel hardware - the most obvious thing is that modesetting has functional VSync and superior 3D performance with less tearing.
Only problem is that we default to intel, modesetting you need to choose explicitly through xorg.conf. I also found similar problems in "radeon", but found that modesetting would somehow pick a display mode that the monitor didn't support. Maybe this is actually a drmkms bug - I'm not sure. But maybe modesetting is mature enough (and intel bad enough) to warrant being the default for Intel GPUs.
