It really doesn't matter.

A RB that runs in the 4.4's is REALLY fast. He's not going to get caught from behind.
And the reality is that short area quickness is such an important asset. Henderson is NOT going to be averaging nearly 9 ypc in the NFL with defenders often chasing him as he heads to the End Zone. Quick moves in the hole, and that sudden acceleration to get through holes that open and close quickly in the NFL will serve him well if he's the guy McVay and Snead think he is.