I don't know that the testing would show you something that really would give you useful information for making the decision.
The success of a Montessori program really depends on the teacher and how it's implemented.
My son went to Montessori and I am a huge believer in the theories. I loved how they were introduced to complex subjects even in the early years (though not necessarily using the true terms). I found myself incredibly disappointed when it ultimately didn't work out.
With my son, the problem was with the amount of repetition required before moving on to a more difficult topic. For example, he had to do an activity where you had to physically place a bar of 3 blue blocks together with a bar of 7 red blocks to see that 3+7=10. He did this daily for months. And when he made a mistake on one page where his mind was just clearly not there that day, the teacher had him repeat the entire book of 100 problems rather than fix the 10 he got wrong. We were having giant tantrums at home because of this, but the teacher saw nothing wrong because he was behaving in the classroom. I got testing to show that he was gifted and hope to use that as proof that he could handle more difficult work, but this did nothing. It showed that he was a visual-spatial learner and I showed the teacher an article about this style, but all it meant was that she "allowed" him to work on the constructive triangles (putting two triangles together to create new shapes, which, yes, is somewhat profound, but a gifted kid gets the point the first time around).
Ultimately, I felt that I had spent $15,000 to have my kid continue to build block towers and count to 10. I don't think that he made any significant academic progress in his time there, and I don't know that he developed the other skills Montessori is supposed to help build (increased attention span, love of learning, etc.).
Many of the gifted kids I know who went to Montessori had a poor time, with experiences similar to my son, particularly after the novelty of the first year wears off. However, it's totally dependent on the teacher. My son's teacher had been teaching for probably 30 years and was pretty stuck in her ways of what children were developmentally ready for. However, some kinds have different experiences, and in retrospect, if I had chosen a different teacher at that same school, I think we would have had a more positive experience overall.
I'll also say that I've seen a lot of people here say that they've had success with putting their kids in a play-based preschool and working on any sort of academic work at home. If we decide to put our younger child in preschool, I think that's the route we'll go.
