They are pretty average when it comes to yards per game and 1st downs (more on that later). I'm just saying when I was digging into stats a few years back, 3rd Down % had a higher correlation to winning within a given game than a lot of other stats including yards per game. Turnover margin had the highest correlation. I can't recall where overall 1st downs ended up but I'm sure its also higher than overall YPG.
If you're awesome on 3rd down, you're going to force more punts (or FGs). Houston is tied for best in the league there. Your earlier stat said Houston has faced among the fewest 3rd downs in the league. Well them leading the league in punts forced per game should tell you a little something about how lights out they've been on 3rd down.
Like I said in my first post, there are a lot of stats to judge a defense. Everyone gives up yards these days, so give me a team that forces turnovers or forces punts (3rd Down %) over a team that allows 2 or 3 fewer first downs over the course of a game. Arizona is leading the league in allowing 17 1st downs a game. Houston is midpack at 20 per game. I'd gladly sacrifice a few yards here and there, or a couple first downs over the course of a game if it meant my defense was going to get me the ball back more often through defensive stops. Change of possession to me is a more tangible benefit and stat than overall YPG or 1st downs allowed.