The problem is this hasn't been against the big teams yet. The true test will come starting 11/11. Texans, @ 49ers, Vikings and Seattle.
I think the Lions could pose a problem depending on who shows up for the Lions. The should win the three games up to the Titans.
My bigger concern on defense is how well they hold up the entire season. I think its great so far but and hopefully the offense can start coming alive in the first half and be constructive in the first half as well.