BTW, the poster over at IMDB who posted the extensive spoilers has this to say about Khan's appearance:
It's never mentioned why he "is white". As I said in another thread, it's just colour-blind casting.
It seems what is important is his physicality, his menace, his lilting, urbane manner, his ability to manipulate, his rage -- in the face of all these attributes his race is unimportant. I presume they lost Del Toro (if they ever had him), at some point they just scored BC, he's cool and current so they went with him.