I can tell you my high school AP Calc class went at the same rate as a typical college Calc class. Maybe a little faster. So right there, my anecdotal evidence in juxtaposition with yours shows you can't generalize from your knowledge.
Was that BC Calc? Because if it was, then that's not really a fair comparison because BC Calc is supposed to cover 2 semesters. That being said, you're not seriously arguing that AP Calc is harder than taking the course in college, are you? Any day of the week, I'd rather learn calculus from an experienced, English-speaking teacher in a 20 person classroom whose career goal is to teach students mathematics than from a math prof who only teaches because he has to in a 300 person lecture hall and a TA from Elbonia. A few decades later, I remember a lot of calc from my AP calc class, but I don't remember a damn thing from calc 3 which I had to take in college.
Anyway, the AP classes that I was talking about were things like AP U.S. History, AP U.S. Lit, AP Chemistry, Physics, etc. These were all semester-long courses in university, but took a full year to teach in high school. Learning the same material in twice the time is not impressive to me.
The school system is a factory system. Factory systems that are tuned to the mean, roughly. Individualized Education Programs (IEPs) set up after NCLB are esentially a selection of 7 or 8 learning supports or enrichments that are minor modifications of the base system.
Aren't we talking about a minor change, here? All I'm saying is that a motivated student ought to be able to take a bloody AP class if he or she wants to, even in the absence of a stellar transcript. I mean, really. If a kid is fascinated with history, why shouldn't that kid be allowed to take AP U.S. History instead of standard U.S. History? The kid'll get some exposure to college level work in a class that piques his or her interest instead of being told, "Hey, sorry kid. You're too dumb to handle college work."
Back when I was in school, there was this chick who really wanted to take AP Econ, but she didn't have good grades, and the teacher gave her the boot (it was in front of the whole class, too. Ouch!). All I'm saying is that there were plenty of supposedly qualified kids in that class who never even came close to grasping the material, but I bet this chick would have gotten it because she cared more than they did.
I know it's just an anecdote and I know it's just my opinion not a scientific study, but anyway, that's what I think. Because frankly, who cares if they fail? I'd rather see a motivated kid challenge him or herself and come up short than not to take the challenge at all.