Ranking quarterbacks based on their clutch factor is much different than ranking their upside. Some quarterbacks perform better in pressure situations, but that doesn't mean they are better overall prospect. However, all of the elite quarterbacks know how to win when the stakes are the highest.
The best example of the impact a clutch quarterback can have is Tim Tebow. His play throughout the game is erratic at best, but in the fourth quarter he finds a way to win. Tebow would rank high on the clutch ranking, but lower on the overall prospect ranking.