The need to have Khan look like a genetically engineered superman is a big reason why it won't be Khan, not played by some scrawny, fugly guy with an accent that doesn't belong with a guy named Khan.
Montelban was physically right for the role because he looked extreme - very handsome, very built, very exotic accent (in a time when the audience wouldn't be expected to know the difference between an Indian and Mexican accent).
That conveyed the idea that the character was a physical manifestation of perfection, before a line of dialogue was spoken. So when he revealed his backstory, the audience was primed to understand and believe it, because he looked it. (The exoticism was to throw in some sex and danger.)
TV and movies are a visual medium, and particularly in this era of global audiences who might be reading the dialogue from subtitles, and not necessarily following very closely or even being very literate, it's vital that anything important be conveyed visually, so that it supports the dialogue and plot. If the visuals are clashing with the plot, the audience will get lost.
Cumberbatch does not visually communicate the idea of a genetically engineered superman. But he sure does communicate the idea of a supercilious egomaniac who may be off his rocker. If the character were Mitchell, I'd expect more of a fresh, youthful, all-American type, an evil version of Pine's Kirk.
I think you people have convinced me, it IS Garth!