I like Cumberbatch, but he doesn't have the pecs for the part.
Ok, so this is a little off topic, but I'll swing it back around...
One of the things that has always bothered me about Superman is how muscular he is. He has super strength on this planet and depending on the era can lift anything. How does he challenge his muscles enough to grow them?
This is why I always liked Reeves' Superman in the films. He looks athletic and definitely had some bulk, but he wasn't freakishly huge/cut as he's often drawn in the comics.
I think this same logic can be applied to Khan. He doesn't have to be muscular to be genetically superior and stronger. He could be relatively thin, but with excellent muscle tone and muscle efficiency that makes him superior.
Besides, I wouldn't exactly see Khan as the type that wastes his intellect pumping iron.
Also, Montalban was definitely fit in the original series, but I don't recall his chest being bulked up nearly as much as he was in the film. This could be explained be the hard work he had to do on Ceti Alpha to survive.
Long story short: Cumberbatch's physique isn't a concern for me. The fact that Khan wasn't an English dude is.
