When I played on DX10 servers after the mod first released, I experienced a noticeable graphic enhancement. The textures appeared to be more crisp and the rockets' smoke trails were certainly more realistic. There is more to DX10 than motion blur, but I'm not sure if DX9 can do volumetric clouds or other enhancements like that. I guess if the devs say they're the same, then who am I to argue... But I'm not sure how to account for the better visuals. There just isn't much population on the servers so I play on DX9 servers.