I do know that my direct competitor has a pagerank of 6 and they are getting 4 levels indexed, while we are 5 and are getting 3 levels indexed.
I believe that answered your question right there. PageRank is like the Richter Scale. The difference between PR5 and PR6 is pretty impressive, especially with a site that is structured with deep links.
I don't think that is totally a coincidence. What I did is put a sitemap of all the past level 4 content into a level 2 page so now it is all level 3.
I believe pages should follow a logical directory/category structure. Within that structure you have root level pages that act as indexes into the content within each category. If the site is large enough, you'd have multiple site maps to control the flow of indexing. Internal linking structure is key in this instance.