IMO, the past 2 years were Coach Saban's best defenses. They were the fastest, and built to combat the HUNH offenses. And yet, we still saw how teams like OM and Clemson, with great QB play, gave these two defenses so much trouble (aided, in no small part, by our own offensive ineptitude at times, but I digress). So I say that 2011 defense, stellar as it was, would have likely struggled greatly against the exact same Clemson teams we saw the past two seasons. JMO.
edited to add: just look at how they struggled against a triple option team (extreme departure from the norm, like the HUNH is), Georgia Southern, with half the talent we have, if that. I know we had a couple of starters out for that game, but still. (We had starters out for the Clemson NC game, too).