Is there a "scanline sync" feature or a way to emulate it?

DXGI_SWAP_EFFECT_FLIP_DISCARD was introduced with Windows 10. Windows 8.1 is still limited to at best using DXGI_SWAP_EFFECT_FLIP_SEQUENTIAL, which I guess might not be as performant as flip discard.

Special K basically makes use of the very latest features of Windows 10 to enable low latency in window mode.