I agree 100%, 'Neato.
I was always a social chameleon--I could fit in darn near anywhere you put me, but I knew myself pretty well (better than most kids, anyway). I see the same kind of thing going on with my boys. Both of them seem to have friends of all sorts. They enjoy older GT kids, but they also have good friends who are younger and/or not GT. They just do different things with different people.
The trick is the not losing one's self in the process, I think. There's a difference between being a chameleon, which I think of as being capable of connecting with all sorts of people on all sorts of levels (as you describe, 'Neato), and being a follower who can't stand up for him/herself. I have seen evidence that my kids have a strong sense of self, so in that case, I think their "fitting in" is a very positive thing. Fine line there, though, you know?
As for the overachiever thing, I hear you, Val. That was me in high school. I felt driven to get straight-As because if I didn't, who was I? I had begun to define myself far too narrowly as my accomplishments. It wasn't healthy.
I read a book in my church youth group way-back-when that talked about "the scoreboard of life," and I actually gave the sermon on the topic for Youth Sunday. The concept that we are NOT the sum total of our accomplishments really resonated with me, and it came along at the time when I really needed to hear it. I really tried to internalize that message: we are more than what we can do, more than what competitions we win, more than what we know.
It's why I think it's important to give kids chances to make mistakes and fail and be lousy at things. It's why I think we parents need to try new things and be lousy at them, too, so that our kids see that it's okay. We can pick ourselves up, dust ourselves off, and start all over again, and it's okay.
