VXGI is meant for any+ card, but certain features (hdr emittance) are better with . With the right optimizations the 770 can easily hit 40 fps in the sifi hallway.
Edit: VXGI also runs significantly worse in editor. In a standalone I got 60 fps on ShooterGame with specular tracing, diffuse tracing,and a map size of 128 with static lighting disabled just by disabling hdr emittance and removing the point lights. Multi-bounce still kills me though.