This is a big discussion in our household too. My DH (who is PG) is good at everything but great at nothing (from his perspective). As a child (and even now), he had difficultly focusing on one passion.
He actually wishes his parents would have evaluated his skills and interests along with the future in the area to help him focus on a few key things. Questions like: Was he ever likely to use skill as adult? Would he ever become a professional baseball player or musician? Could he get a job as a physicist?
I know the book Smart Boys by Kerr & Cohn addresses this to some extent too. (I'm paraphrasing here . . .) They find that gifted men in the workplace never achieve eminence because they were never taught to focus as children. Typically, people focus on the one thing that they are good at- since gifted children are often good at lots of things, they never focus and fail to develop a true passion.
Now, how you go about deciding when and how to cut activities? No clue. Sorry!
