They weren't born quick, but they can be quicker. Of course your leg length plays a role, but Tyson Gay is only 1m80. He's yet another prime example of a sprinter who has more hamstring development than quad development. You're right in saying that the upper hamstring is what you should work on as it is vital in hip extension, but if you're going to counter this by building huge quads, none of your work will matter. Look at how Amauri and Diego are running up that hill. Does that really look like proper form to you? Seems like they're wasting a lot of energy there and considering the fact that our team has had a gazillion hamstring injuries in recent years, I say we should change something.