Witten seems to be getting his catches, and I'm going by impression, when the progressions on WR routes are covered. He is Romo's outlet, the implication being, he isn't the primary target and the defense is ceding the middle of the field during those times. It is not like Romo is always looking for Witten, it is because teams are funneling and have been funneling the passes like that.
I don't think Witten has the speed anymore to attack down the seams, but the problem is we don't attack the seams, but rarely, ever since Garrett's play book. We seem to be doing it more though, but Dallas needs to start moving in direction beyond Witten.