My own personal experience from three years of being an undergraduate math instructor:

Typically during the first week of class, around 10% (six or seven students) will approach me asking if they really need to take the class if they got N on the AP Calculus exam, where N is usually greater than 3 and they rarely distinguish between AB and BC.

I encourage them to stay, because they're usually misjudging the difficulty of the course from the first week or so of classes. I can't recall any of them doing well. There's a lot of confounding factors in this account, however....