From my point of view the deep forum structure will not lead to more contributions than a flat structure. With a flat structure people can post topics even if their post doesn't fit in 100%. With a strong hierarchical structure there is no "air to breath" for thoughts and ideas. From my point of view - which is over 10 years of professional website building - a flat structure drives the people to ask and answer.
I suggest to delete at least the deepest level and make sure that we have many terms in the first level.