Just does not work. With X4250.kext, Quicktime is still using the 3770K CPU to play the HEVC Sony sample (IGPU is disable in the BIOS).
Try temporary disable shiki, then clear DRM and GVA config, rebuild dyld_shared_cache, as below.
defaults delete com.apple.coremedia
defaults delete com.apple.AppleGVA
sudo rm -rf /Users/Shared/SC\ Info
sudo defaults delete com.apple.coremedia
sudo defaults delete com.apple.AppleGVA
sudo update_dyld_shared_cache -force
Then re-enable shiki and reboot.
I use above method in 10.13.2, no VCPHEVC show up while high bitrate 4k@60 HDR HEVC playback, only AMD HEVC decoding. See sony2 png without IGPU
If IGPU can not disabled in BIOS, then use shikigva=1, it will disable IGPU GVA in dual GPU system. See result in sony1 png with disabled IGPU
See attached pngs. Both VTDecoderXPCService CPU usage show up round 10%