FINALLY.
I'm excited about this. Right now, I run 10bit 4K HEVC videos in Windows via Bootcamp (MPC-HC) and it's buttery smooth, compared to the stuttering in Sierra.
I have a MBP 2016 15" AMD 460, so the dGPU does have support for it. I hope High Sierra takes full advantage of the dGPU for HW support.
That has nothing to do with the Operating System.
AFAIK when you run Bootcamp-Windows, your iGPU is deactivated and you are running on the dGPU as only GPU.
The dGPU is able to decode 10bit-HEVC, but the dGPU will consume more power.
The problem in Sierra is that AFAIK there is no software that is able to accelerate the dGPU to play video. e.g. AFAIK VLC does not support to play 10bit-HEVC with dGPU and tries to play it with CPU only.
Maybe High Sierra will change something on that side, but it will result in higher power consumtion for the Skylake-models.
On the Kaby Lake models there is 10bit-HEVC Hardware en-/decoding, so there it will always run on the iGPU.
=> more battery efficient, lower temperatures, operating quieter (fan speed)
8bit-HEVC is possible on Skylake and Kaby Lake.