I don't know about that adrenaline argument. I still play ball and I don't recall the feeling of being chased as a factor in me ever outrunning a guy. In fact, getting hyped up can actually have more negative affects when trying to run fast. It tightens up your upper body and doesn't make you as fluid. The fastest sprinters try to stay relaxed. Watch their jaws when they run - they flap around rather loosely. I recall practicing good sprinting form (elbows bent at 90 degrees, relaxed upper body, etc) as a part of regular football practices going back to middle school. Training athletes to get even 1% better with their running form could make the difference between scoring on a play and getting stopped at the one.
I am currently training at the Michael Johnson Performance Center in McKinney, TX and you wouldn't believe how many NFL players (not including rookies who were just drafted) use their services to learn better running form in the offseason. I had the pleasure of working out with a couple of them this spring and summer, and it was a blast. To them, it is all about making many small gains to add up to a larger gain over their competition. The coaches and trainers there are complete technicians. These are players who are focusing on being more explosive and having better technique, and are NOT training to run the 40. They are training to be FASTER. On of the things my trainer always tells me is to simply stay relaxed. I have a tendency to get too 'amped up' before a sprint, especially in a timed session. It makes me less fluid and tight, which hurts my flexibility.
Sprinting is all about stride length x stride turnover (RPMs). Being loose allows you to be more fluid. Having too much adrenaline and not being calm can hurt that. These are pretty simple principals.
The other thing you mention is about scouts using what they see in games to measure speed. I don't find that to be true at all. I have written and read thousands of scouting reports in my day, and very, very little of a player profile has anything to do with their speed in the game. What will typically be listed is their 40 time and their splits to represent their potential for speed in ideal conditions. Scouting reports that contain a wealth of subjective data instead break down the technique of a player. They use this analysis to determine if that potential for speed transitions well onto the field, but it never determines if the player is actually slower.
In football, I would say that the majority of what is important for every position is technical, not athletic. Sure, being superior athletically helps, but it doesn't mean that a player will be successful. Look at Troy Williamson. He was a burner, but couldn't read coverages or run any route aside from a 9. That doesn't mean that he was slower though. It means that he didn't have the technical capacity to realize his athletic potential on the field.
The same follows for guys like Jerry Rice. He ran a 4.58. Not a burner, but his technical skills were among the greatest of all time for his position. Watch the old cutups of him running routes with the 49ers in the 80s at practices. DBs could backpedal as fast as he could run coming straight at him. However, he would take a cushion of 1 yard at the top of his stem and turn it into 5 yards out of his break. His ability to run routes at full speed (albeit, 4.58 speed) is what made him special. Most WRs don't run routes correctly. They slow down too much. They don't utilize pressure steps.
Kids at The U weren't even learning them. I was privileged to attend a coaching clinic at UW-Madison in 2006 and I was shocked to find out that the WR coaches at Miami didn't even teach pressure steps. Instead, they relied heavily on crisp, choppy, but slow, cuts. Using pressure steps will make you faster in your break, but will make your cuts look rounded at the top of the stem. Some guys don't like that, but it is the best way to run routes at higher speed. It gives the DB less time to react and doesn't give you as much of a chance to give the route away.
I know I have digressed a lot from your original comment, but in reality "game speed" is what people erroneously use to lump a multitude of technique-related skills into one category that they can't otherwise explain. They see a "slow" LB like Zach Thomas get to a ball carrier who is way faster than him in the hole and think that some magical power or adrenaline made him get there faster. However, what the amateur doesn't realize is that Thomas probably saw the play a hundred times on film, found a key to the play (play recognition), and got a jump to the hole before the ball carrier could hit it. He wasn't faster than the ball carrier; his instincts and superior recognition skills allowed him to get to the spot very efficiently...even if he only possessed 4.80 speed.