yeah, i read that, too.
but that is only a useful information if you already have a directx11 gpu and are running wow in directx9 mode.
obviously an older card cannot benefit from directx11 mode (i don't exactly know if directx11 commands are emulated via cpu then, i doubt a performance gain in that case).

so if you have a powerful dx10 card - well... then you have a powerful dx10 card, which runs wow in dx9 mode (there is no dx10 mode that i know of).

a dx11 capable card should be run in dx11 mode, though. always a gain, up to 20% if you have a lower end cpu - or perhaps if you are multiboxing. but i think there are far more knowledgable people about this specific topic on this board than me.

all in all wow stays cpu heavy, so everything a newer card would do, would be to enable you to run a bit higher graphics settings on your clients. i say "a bit higher" because the most limiting factor is and probably always will be the cpu, rather than the gpu. that's why a strong cpu is always a better upgrade as a strong gpu.

bad implementation on blizzard's side, i guess. only guesswork, though. i'm not that much into game programming in general and have absolutely no idea of the wow-sourcecode, so i couldn't decide if this could be solved better.