For those who're interested:
Shadow Warrior was updated yesterday with a DX11 patch.
Not only does it greatly optimize the game performance-wise, and even in single-card performance at 1800p I now have a continuous more-than-100fps, Crossfire (as in actual POSITIVE scaling) now works.
Just make a profile for sw.x64.exe (no not the standard sw.exe, that one doesn't work for the 64-bit/DX11 version), and set CrossfireX-mode to 'AFR-friendly'.
I had to DISABLE the in-game 'Tripple Buffering'[sic], or it's laggy using CFX. And yes that's odd since usually one has to actually enable it to reduce input-lag...