I can see why you would want to add XF as the forum, however, since the forum there is clearly nowhere near like regular forums. I guess that must be the kind that comes with Drupal.
Where Drupal forums lack, they also excel in a way traditional forums lack.
I will try to explain:
Every time a new Drupal forum thread is created, it is really just another type of article. Once you understand this (what articles and pages are) these can share everything within the CMS structure (and be programmed to include more of the CMS as well)
Meaning, forums can be expanded into articles that share keywords (see taxonomy) that can be programed to autolink other content by means of direct links via menu's, keywords within the contexts of all related content which can be programmed to open or close windows called blocks wherever you want shown throughout the CMS.
Every thread (forums, article, pages) can be dedicated to just forums or full pages that can trigger images, blocks that can appear in a multiple array of sidebars, headers, footers etc.
Everything you create can be turned on or off depending on how you want something to appear.
The possibilities in how you want things to be presented are really unlimited.
Each member can have one or all of the permission granted. The CMS becomes a big big tool to build content. Forums are then, just a way to string members threads together, that can also be shown past the forum module.
xenForo would be one of many ways to create and share forum content within a larger main frame sharing its data with the CMS search engine.
This is when it gets exciting.
PS, all of what I mention can also be visible/ invisible on a user permissions setting. Meaning, if you don't want the public or a group to see area's of your CMS, its a simple checkbox.
Also, What one user see's is up to the next user. Everyone has the ability to use the CMS they way you want or allow.
. Its never been updated since I bought the product. It looks good though, which shows there is potential. 
