I like the Vader audible reactions, it shows he still feels some pain, even being as much machine as he is. Remember, Vader is not meant to be person who is all powerful, he is a flawed person who didn't stay with his potential.
But go back and listen to the original pre SE editions of the vhs tapes or maybe even the pre SE dvds (Which I don't have.) Vader's Carbonite groans sounded like two different ones, in the SE's they both sound more or less the same.