The are not in the ballpark with the 2000 team. But as you mention, not many teams in history have been.
The defensive line will not be able to dominate the way that team did. Even if Ngata grows up in a hurry, he and gregg are still not ready to overpower people, control the rushing lanes and disrupt the offense. Getting price and Ngata to replace Weaver and Kemo isn't a huge upgrade when you consider the age of one and the inexperience of the other.
After the starters, the defensive backfield seems to drop off quickly.
There are a lot of tweeners on this team (particularly DE/LB) so it will be interesting to see if this results in more flexibility or liabilities teams can exploit.
Even if he can stay healthy, which is not a given, Ray Lewis is simply not Ray-Ray anymore.
I think you did touch on one of the keys however.
If McNair and the offense can sustain drives (and score some points while doing so) I think it will help the defense immensely in terms of rest, confidence, and putting pressure on the other team to take chances (and thus make mistakes).