Great assessment Kaos, and right on, obviously.
Aside from speed, I don't think size can be underestimated as an advantage to them as well. About a month ago or so I watched that USC/Oregon State game from last year. Jacquizz Rodgers was unstoppable in that game. He makes Devine look like Brandon Jacobs. See, not only are they lightening-quick, they run extremely low to the ground and give very little "body" to target for true tackling. You can't simply hit them high, because they have such balance. And going for their legs is quite a task because they are so low.
And it's not just Devine, it's the main three guys that ever had the ball outside of Brown (who is like trying to tackle Vince Young (in college). Not that he ran like VY, but he was every bit as elusive and slippery.
We spent a lot of time chasing them, but we also spent far too much time simply throwing ourselves at them trying to "hit" them. Roof wants a defense that is aggressive, physical, and punishing. I bet this week he emphasizes a few more tackling basics this week to go along with all the "hitting".