Just wondering if your child has any issues with fine motor skills, copying either far point (from the board) or near point (from the desk?) The difference between his thinking scores (VCI and PRI) is ENORMOUS and could be a red flag. He is a "Ferrari" that is being powered by a lawn mower engine. In time, this can cause some significant frustrations. I am hoping that the score is somewhat skewed by his approach to the testing, However, his "approach" shouldn't be discounted if it is something that comes into play on other academic tasks. (the chicken and the egg - is his approach due in part to the large discrepancy in his cognitive profile, or did the testing show disparate scores due to his approach). What is the root cause of the quick, impulsive approach?
I am wondering if there are some fine motor, visual processing, executive functioning or attention difficulties coming into play. And, if there are, this could help you address whether or not Montessori would be a good match.
Montessori can be very good for a self directed learner that can generalize what they know to new situations. It is often hands on and multi sensory, which is good for almost every learner. Most Montessori schools have multi aged classrooms which can be great for younger, bright students that benefit by working along more advanced peers. And the older kids have an opportunity to work on leadership skills with younger kids. The program is child centered - and I believe this is always a good thing!
However, if a child is not able to set internal goals, independently apply new skills to new situations, requires explicit, sequenced instruction with frequent skill checks to learn, it may not be a good environment for them. Some of the issue I mentioned, especially Executive Functioning problems could hinder a child's growth in Montessori.