Perhaps the line needs to be drawn individually for each kid. For us, DS was never accelerated as fast as he was able per testing results - we actually made an affirmative decision to choose a less radical path. The compromise was to accelerate DS enough to prevent harm from destruction of the "spark".
For DD, who is not mathy and who will not study AP Calculus AB until 11th grade, we chose not to accelerate at all. She actually could have handled Pre-Algebra easily in 5th and we knew that by 4th grade. However, she tends to learn everything quickly although not in a deep way. In her case, I can see her mastering the procedures immediately and acing everything but perhaps missing the deeper connections and ultimately vulnerable to potentially forgetting down the road.
