Vega's HBCC is apparently tied to WDDM on W10. So currently no W7 or W8.1 support.
The problem with this is that W10 has many, many performance issues, not least in gaming.
They tend to cause major stutter, frame drops and general instability.
HBCC is designed to reduce stutter and micro stutter significantly. But currently in order to use it, you must use an OS that is absolutely plagued with it, and unlikely, in my view, to be fixed.
More and more people are rolling back to W7 (and 8.1) to avoid both these issues, and serious privacy concerns.
Can we please see HBCC implementation on Windows OSs other than W10. It's just not a viable platform for gaming at the moment.
It's not as if WDDM is absolutely necessary, anyway ... it's been confirmed that HBCC is being worked on for the open source version of Linux Radeon drivers.