Its NOT speed its acceleration. The guys the get my juices going are the ones who have spectacular 3 cone times. That's a drill that shows how well a player accelerates. Its a much better indication whether a player can get separation. IIRC Deion Branch had a very pedestrian 40 time but the 2nd fastest 3 cone time.
Also, I read last week that there was a measurement that they can take during the 40 that is a much better indication of how a player will project. I can't remember exactly what it was, but what I do remember that while Jerry Rice had a very disappointing 40 time, he was the 2nd fastest in NFL history in this measurement that I can't remember.
I'd appreciate it if someone with a better memory can help me out here.
BOTTOM LINE - its not about the speed, its all about the acceleration.....body control, hand/eye coordination, and hands.