How sad that we have even have to worry about transitioning to a traditional school - "traditional" meaning there is potentially no self-directed learning, no conceptual coaching, no individualized achievement and no respect for the child.
Montessori is absolutely the way to go. Even if your daughter has to "dumb it down" to survive in traditional school, she won't be any worse for having a Montessori background, and no doubt she will be better for it. The biggest problem she may encounter is having tasted the relative joy of self-directed learning, she won't like the taste of traditional (teacher directed) school.
P.S. If you can keep her in Montessori longer, do it! Both my boys have been in a Montessori school since pre-school, and I could never send them to a traditional school. It is amazing what they know (and they don't even know they know it).