I don't know enough details of Steelers games, but it seems many points they gave up were from special teams (KR/punt returns) and turnover? I recalled a fumble return for 6 in the Denver game?
Take those points out and the Steelers D is even better. We have another measure for defense that is based on scoring efficiency and susceptibility to long drives. With respect to these, Steelers are very very good. (I didn't try to rank them). Ranking #1 or #2 aren't mattered. They are meaningless since the capability of an offense or defense is multidimensional. It's like trying ranking people in totality: look, intelligence, personality, charm, integrity, friendliness ...
Let's just say that your Steelers D does look very good (even discount the Miami game) in the few that I saw.
Good luck.
I still think the Pats will put 11, 12 drives on your D, 4 TDs + 2 FGs or 3 TDs +3 FGs.