Intuitively, it seems more effective to educate children one on one. I wish I had received my formal education in a Montessori school, I know I would have developed into a much different kind of person. Of course, financial constraints are the reason society does not educate everyone this way. Furthermore, It is unfortunate that we cannot better quantify the Montessori method because as stated in your article:

Absent traditional forms of assessment, it is challenging to draw comparisons between Montessori programs themselves or between Montessori based programs and others.

More research on Montessori programs is definitely needed.