Many people here have a lot more background and far more expertise than I do. And as I said, there's as many opinions about SEO best practices as there are people that think about SEO best practices it can be a murky field.
The main thing to keep in mind about SEO is that gaming the system is almost inevitably going to be a mistake in the long run. Focusing on quality content, in a logical site structure with lightweight, semantic html is the way to go. XF happens to be a pretty dang good platform for that purpose, so I fully expect an XF based site that focuses on good content to do very well. The main reason many Forum sites add a CMS is because it plays well to Google, or because they have seen other successful forum sites do so.
Personally, I think that "Must have a CMS" attitude is often a case of confusing the symptom with the underlying cause. 
Good content, well presented. That's the secret IMO. Doesn't matter what form the content is in as well as it's well formed semantically presented well and most importantly contains unique, information that's highly relevant to the topic.
For me, it is pretty well established (based on my experience and my readings of Matt Cutt's blog etc) that dividing up your content into separate systems is not optimal.
