This would be a good idea, the only problem I can see would be windows xp compatibiliy because it directX 11 was designed for vista and 7, it could be comatible with xp sp3, but im not 100% if it is then they really should do migrate to 11.

In FFXI people with newer graphics cards are seeing lag problems because their cards dont support directX 8.1, and unless they do keep using directX 9 this problem will happen very fast, and it will be alot better for SE to implement a directX 11 upgrade asap. rather than wait.