Both Football Outsiders and Advanced NFL Stats show the Bills are on the verge of having an elite defense, despite what the yardage/point totals might say.
Football Outsiders has the Bills ranked 6th in Defensive DVOA at -9.4%(negative numbers are good on defense), trailing on Seattle(-22.3), KC(-19.3), Carolina(-14.3), NY Jets(-12.8), and Arizona(-10.0). Bills are ranked 4th in Pass Defense at -9.2, trailing only KC(-32.0), Seattle(-24.9) and Carolina(-11.1). Bills are a little lower in Run Defense ranked 14th at -9.8, which is still pretty good. Without adjusting for strength of schedule and opponent, the Bills actually vault to 4th best in the NFL at (-12.5 total, -13.8 pass(4th) and -10.8 run(11th))
Looking further, the Bills have had the most defensive drives against them in the NFL with 96(average is 78) and have played very well. They rank 8th in average yards given up per drive with 28.04(last year 32.73(26th)). 12th in points per drive with 1.71(2.23(29th)). 10th in turnovers per drive with .156(.112(24th)), 4th in INTs per drive with .125(.067(21st)), 10th in plays per drive with 5.47(5.93(19th)), 9th in time of possession allowed per drive at 2:26(2:40(16th)) and 7th in drive success rate at .655(.727(31st))
Advanced NFL stats has the Bills even higher, ranking them the 5th best defense. In defensive expected points added, the Bills are 5th at -11.5, trailing only KC(-56.1), Seattle(-41.3), Carolina(-27.6), and New England(-21.7). In defensive success rate percentage, the Bills climb to 3rd at 58.2%, trailing only NY Jets(60.8%) and KC(59.5%). Bills are ranked 6th against the run(run expected points added) at -18.4, trailing only NY Jets(-28.6), Tampa Bay(-21.7), Philadelphia(-21.7), Green Bay(-20.1), and Denver(-19.9). Run success rate isn't so great, at 59.3% tying us with Carolina for 14th. Passing success rate ranks us 3rd at 57.4% behind only KC at 60.3% and New England at 59.4%...
Also, individually, NFL stats has Kiko Alonso rated as the best linebacker in the NFL currently by quite a wide margin, with a massive 1.82 win probability added(WPA). The second best linebacker is Levy from Detroit with 1.16. Alonso also has a huge lead in expected points added(EPA) at 50.2 over Levy again in 2nd with 38.1....3rd place in this category? Poz! at 37.9. Alonso also leads in Success Count with 52, placing 5 ahead of Navarro Bowman who has 47.
Mario Williams leads all defensive ends in WPA with 1.23, with Carlos Dunlap of Cincy in 2nd with 1.13. Williams is 10th in EPA with 21.0 and is tied for 16th in Success Count with 19. JJ Watt is the runaway leader in this category with 45.
Kyle Williams leads all defensive tackles with a 1.26 WPA, a wide gap between him and second place Suh with Detroit at 0.87. Marcel Dareus is 4th with 0.80...Suh leads in EPA though at 28.6, with Kyle placing 3rd behind Suh and Hatcher of Dallas(24.1) with 22.8. Dareus is 9th at 16.3. In success count, Bills have the top 2 defensive tackles in the NFL. Dareus leads with 36 followed by Kyle in 2nd with 32
Unbelievably, the Bills have another player rated as best at his position thus far in Win Probability Added(WPA) and that is Searcy at safety at 0.98, leading Eric Berry of KC who comes in 2nd at 0.91 and Reggie Nelson of Cincy who is 3rd at 0.89. Searcy also leads in expected points added with 30.0, again leading Berry who is in 2nd at 26.1 and Kam Chacellor who is in 3rd at 24.6. Searcy finishes 10th in success count at 20.
The Bills also have a top 10 appearance at CB, but it is Jim Leonard(0.64), who should be at safety, however, the Bills have 4 of the top 20 at CB WPA with Leonard at 10, McKelvin tied for 12th(0.58), Robey 16th(0.54) and Aaron Williams(0.47)
So to sum it up, across the board in virtually every metric, the Bills are a vastly improved unit, bordering on elite.
Last year, the Bills only top 10 player in WPA was Jairus Byrd who ranked 4th. This year, the Bills have 4 out of a possible 5 players leading their positions in the NFL(Williams at DE, Williams at DT, Alonso at LB and Searcy at safety), which is pretty amazing considering two of our best players have barely played in Byrd and Gilmore.
Not only is Alonso the runaway leader for defensive rookie of the year, he is making a strong case that he is the best LB in the NFL this year(not even close based on advanced metrics, he is Peyton Manning like ahead of the next best player at his position currently) and making a case for defensive PLAYER of the year.
Mike Pettine has been a God send to this defense and it cannot be understated how good of a job he has done. To put use your players so well that 4 of them lead their positions on defense in the entire NFL is one of the most amazing things I have seen in a while...