I'm slowly coming around to the idea (advanced in another thread) that not only is Cumberbatch not Khan, but that it really doesn't matter who "John Harrison" even is. Storywise he's Darth Maul, General Grievous, Bane, the guy Robert Carlyle was playing in The World Is Not Enough.
In other words, he's a distraction. A red herring. A decoy. A henchman at best. The REAL bad guy (girl?) of Star Trek Into Darkness is someone we haven't met yet, someone the prerelease marketing isn't even hinting at.
Lately, I've been having that same thought myself. Either way, doesn't matter to me as long as we get an awesome movie out of it.