goo.gl/Wif3nS
I did some 3D testing and eventhou there is not nearly enough data to confirm it, I'd say the SMT regression is infact a Windows 10 related issue.
In 3D testing I did recently on Windows 10, the title which illustrated the biggest SMT regression was Total War: Warhammer.
All of these were recorded at 3.5GHz, 2133MHz MEMCLK with R9 Nano:
Windows 10 - 1080 Ultra DX11:
8C/16T - 49.39fps (Min), 72.36fps (Avg)
8C/8T - 57.16fps (Min), 72.46fps (Avg)
Windows 7 - 1080 Ultra DX11:
8C/16T - 62.33fps (Min), 78.18fps (Avg)
8C/8T - 62.00fps (Min), 73.22fps (Avg)