I did not review the AZ game for Sullivan in detail, but my impression was that he was doing fairly well. i saw him make a tackle or two. Certainly much better than the Falcons game, where he was frequently getting chop-blocked, without ever getting a flag.
The rush defense was very good against AZ (last year #32) versus against Atlanta (last year #1).
So you choose how much is just due to who you played.
In fairness, there was no game- planning against QB scrambling. Atlanta's rushing stats last year were inflated by their QB rushing stats, and the Pats did give up 65 yards to the Falcon QB runs. But they also gave up 29 yards to Leinhart runs, too.
Sully can do it, if they can ever build a little fire under him. I think he is not overweight any more; he looks like he is comparatively svelte and NOT a tub of goo. Conditioning may be another issue though. Maybe Pepper can motivate him or the looming end of his money gravy train might...