I believe that the success of a young HG+ child during a particular year of school is based primarily on the specific teacher. If the teacher "gets" your DS, it will be OK. If the teacher doesn't, its a lot less likely to work... Which is why, as ConnectingDots indicated, Montessori works great for some and not for others
Totally agree with this. In theory Montessori is great for gifted kids (i.e., follow the child and move at their individual pace). However, not all Montessori schools/teachers operate in the same manner. If a Montessori teacher (that is true to the philosophy) "gets" your child, then it can be great! Especially since students stay with the same teacher for 3 years. But imagine being with the same mismatched teacher for 3 years! Oh my!
There are, however, some philosophies that just don't mesh with certain kids. For example, I'm pretty sure my child would implode if placed in a Waldorf school. But if the method seems to be a reasonable fit, then I agree that the teacher's understanding and flexibility are the primary contributors to success.