No, I think if they had Kai Leng mugging and monologuing like some cheesy comic book villain it would have been painful. Besides, he's not the main villain (because there really isn't one when you think about it) he's just a thug who happens to be a xenophobic psychopath...with an N7 rating. I actually found the little video clips of Leng in the Cronos Station level quite telling for his character. He feels threatened by Shepard and unless I'm mistaken, TIM actually respects Shepard more than Kai Leng.
Oh, and the asari scientists with their throats slit on Thessia was a nice callback...Actually, given the history, I wonder if they'll include Kai Leng in the Omega DLC? Not as (another) boss battle, but just there as a character.