I ran the numbers again on the Pats and Colts. Looking at all regular season games from 2001-2010 for NE and all regular season games from 2002-2010 for IND (the COlts were pretty bad in 2001):The Pats defense allowed 17.95 points allowed and 318.2 yards per game.The Colts defense allowed 19.88 points allowed and 320.8 yards per game.That's a difference of 1.93 ppg and 2.6 yds/gm. I'd be interested to hear how that slight difference somehow made the Patriots defense that much better than the Colts. Both teams overall were consistently good record wise in all those years, so it's not like either team threw in a bunch of clunkers.
Looking back over the last couple years, it looks like 18 ppg allowed generally gets you into the top 4 of this stat, while 20 gets you around 10th. So it looks like the 2 ppg is the difference between a very strong defense (elite might be too strong a word) vs a slightly better than average defense. I mean you're talking top 12.5 percentile, vs top 31.25% percentile. Looking at yardage I don't think is very meaningful, based on way defenses play. Some are successful because they just don't let teams move the ball (Pittsburgh is probably the best example of this), while others take a bend but don't break approach where they tighten up aroung the redzone and hold teams to FG's.