If the extraordinary level is really only because all the kid's focus is going into one topic, rather than a greater ability/interest in learning overall, this could be true. It's the pushback I get over asynchrony: if you have a kid who is 2 years advanced in one topic but a year behind in every other topic, they're just unusually focused, not gifted. Offering challenge in the lagging areas, and letting them drift in the advanced areas, brings them into better balance while keeping them engaged. I have no idea how common this actually is, but it *sounds* plausible. I certainly don't know anyone like this.
The 'plateau' word gets used mostly by classroom teachers who aren't able to keep up with the diversity in their class. If your child moves on to art or music or the war machines of the Renaissance, it *is* a plateau as far as their reading/math/other instructional level is concerned - and that makes them easier to teach in a group. As far as the classroom is concerned, this is the same as the first situation (and keeps educators from having to acknowledge the scary, non-PC degree to which *people are different*).
It also happens when new material is persistently withheld from a child who is ready for it. The cynical side of me connects this to the 'learned helplessness' description on another thread. By third grade, a child who can't get appropriate level material no matter how hard they try will stop trying altogether and subside into the academic middle or bottom of the class.
I've found that 9 is a magical age in my family: many of us have very serious fit issues by then resulting in depression, poor sleep, poor eating, lack of memories from the period, anger and acting-out, etc. that are only resolved by a change in school and more appropriate placement. So far it's hit six members of my family over three generations. I hear the same story from many mid-year, emergency applicants to the school my daughter found to be more appropriate.
