I agree they haven't really been tested yet. I think they also have one of the easiest schedules in the league this year though. If they are for real, then 15-1 or 16-0 are distinct possibilities barring injuries.
I watched them a bit last year on their initial undefeated run, and they were good but not superb even when playing weak opponents. This year they seem to be scorching hot, and incredibly well balanced on offense and defense.
In the NFC, only the Packers are in the conversation, and while I might give ARI 9/10 grades on both offensive and defensive ability, the Packers I might give 9.5/10 (10/10 with Jordy) on offense, their defense is not superb. It's merely good.
In the AFC, it's the Patriots all by themselves. The Steelers looked great on offense and mediocre on defense, and now they are out QB1 for a while. The Broncos have arguably the most punishing defense in the league, but their offense is in no way close to teams like the Packers, Patriots, Cardinals, or (healthy) Steelers.
The Packers schedule is also pretty weak. Seattle is a 2nd tier team this year, and I don't see any games GB should even worry about other than Denver and Arizona, both away from Lambeau. Home field will be immensely critical for the playoffs with that kind of advantage. If GB has to go to Arizona, I think their chances are dramatically less than if the reverse was required.
The crappy thing about this is with the cakewalk schedules of both Arizona and GB, we won't know probably until December who's got the goods to make it down the stretch.