Ours was in Montessori (officially, the third year of the primary cycle). It was very good. I suspect that had we put him in the local public K, the first month or two would have been fine. When the novelty wore off, not so good. Purely a guess based on the first grade experience.
We are about to have our first more traditional K experience with YDS (not testing, guessing maybe HG, even EG?). Fortunately, the teacher previously taught a higher grade level and the school is expecting that challenges may arise, based on his sibling's giftedness. So many variables are at play with children, though. This child has been far more eager to take on the art, craft and play aspects of preschool than his older brother was at the same ages. I'm guessing we'll have a grace period. Then again, he's starting to read and the pace of fluency is increasing.
I really feel for you, I am so frustrated by all these arbitrary age-based rules. My best advice is to do what you are doing... research, advocate, look for backup options, remain flexible.
