I don't have time to say much right now but just wanted to emphasize how important the particular school, and individual teacher, are with regard to how adaptable the montessori environment is to a particular GT kid. We've been very lucky so far (knock on wood!!) in terms of teachers - three of my kids are in montessori at the moment, only one of whom has been tested (ironically, that's the teacher I expect to have the most trouble with down the road; DD7 is so "good" in class that I'm not sure how things sit with works she wants to try vs. works she is allowed to try). Both my DS5s have teachers that happen to be perfectly suited to their personalities - one is more 2E, and his teacher is thrilled to be able to show the other kids that in spite of his other difficulties, he does more advanced math works, etc. and most importantly, when he struggled with simple works she let him try a more complex work and he could do it. She's very open minded. And my other DS5's teacher figured out that he is underachieving for his ability, in spite of the fact that she thought he met her requirements for starting K last fall, and she is working at fixing that (especially next year in K); a component of her plan involves the fact that there are apparently other bright boys in the class, including at least one who is very bright (not that I have any way of knowing how bright, but they are all working above grade level, and she is aware that if the class was entirely mediocre, DS5's perfectionism issues would have him sink down to a mediocre level).
Plus, two of my kids are visual-spatial learners, and the montessori materials, as well as the manner of receiving lessons, is far superior to a more traditional classroom for their current situations.
When DD7 was tested about a year ago, the professional advice we recieved was that full-time gifted programming was the way to go, montessori was a close second, and a pull-out program a far distant third. They also advised us to reconsider the educational situation on an annual basis. And we'll continue to do that.
All right, I'm at the end of my rope here, gotta run; hope at least some of this is coherent...