It really gets hard to make comparisons between college and pro players for more reasons than just ability. Many good college QB's never get to see the light of day, because they end up as a back-up/trainee behind someone like Kurt Warner, Eli Manning, Payton Manning, Drew Brees, etc. It is rare that they even get to practice with the number one receivers and O-linemen on their pro teams. It is even more rare that they get to showcase their skills in a game with the first unit. Because of this, many begin to have lingering doubts about their own capabilityes, and their confidence begins to fade. Remember the biggest difference between pros and college is in the head. Confidence is the biggest factor in whether or not you make it in the pros, among players of similar ability.
On the opposite end are those who are so badly needed when they are drafted, that they are thrown into combat immediately, and are just not ready for it. Most don't have the opportunity to grow up in a pro QB household like the Mannings did. There is sooooo much more that they have to learn at the next level that was never considered in college. Many of these types also get their confidence shattered right away, and some never get it back.
So I would say that OPPORTUNITY to be in the right place at the right time has a great bearing on how a college QB fares in the pros. While there are many ways to get help, there are many more to get hurt by the position you are drafted into and the opportunity you have to learn or to be forgotten, or even thrown to the wolves.