The difference between DX9 and DX11 will not be all that much. The main improvement in DX11 vs DX9 is increased sunshafting, water-effects, tesselation. While adding a new client to enable DX11 call use will help these things it has two things going against it for an MMO. First is over all most people will barely see any performance boost and actually many who will enable it just because they hear it's better will have a performance drop. There is also the thing of if square even bothers optimizing the game around the newer API.

While in the end options are always nice due to the difference in systems out there, you have to stop and ask if this one is of most importance over other things about the game that could be worked on.