So much also depends on the teacher assigned to the class, and whether or not his/her teaching style matches up with the kid’s learning style. And on the course itself… Geometry is an entirely different animal than other math courses, and kid’s grades in Geometry can sometimes be radically different from the rest of his or her math grades.
And let’s throw in the kid’s attendance record; I’ve had some honors kids who are absent a lot. (I’m not doubting the cause behind the absence; I’m not the attendance police. But every class the kid misses is one class further behind he or she is.)
In my experience, kids who move down, say from Honors to ‘regular’ math, tend to get the exact same grades they had. Once you hit the upper level math courses, it becomes more about things like study habits and work ethic than about the ability to keep up and understand the material.