Historically, I'd say we've overachieved. It's pretty obvious - winningest franchise, with 4-5 non-winning seasons in franchise history! And we haven't had that much talent all the time...
However, lately (personally, it all started in '98) we've underachieved. We've been blessed with great drafting defensive-wise, and had *enough* (yeah, that's debatable alright!) offensive weapons to let the defense takes us all the way (if that makes any sense). Now, the reasons... that's a whole different story.