I understand this may not be an issue with forum only sites, but wouldn't this make xenforo SEO-wise kinda less suitable for CMS integrations?
For a FORUM the idea is brilliant, because the most important content (the threads, posts, etc.) are loaded first to let visitors start reading even before the header (with it's sometimes very heavy graphics) is loaded.
This may not make a lot of a difference at a small site, but for a large, very busy site this is a must. It is one of a few reasons we decided to use XenForo. We had a similar approach at our old vb forum, but it was very difficult to create and update.
Consider adding the category tabs under or above the breadcrumbs. In other words, modify the templates, set up a div for your categories, and disassociate it from the Header, so that you can move it wherever you want (i.e. above the breadcrumbs).