Having the content in the forum is better for SEO? Hmmm sounds interesting. Tell us more. Why not just use Wordpress and tack on XF?
Obviously there's a lot of different feelings about SEO best practices. And in my experience a well implemented Wordpress implementation ranks very well with Google indeed.
Personally though, I feel that a lot of that effectiveness is because WP (like many blogs) has an excellent basic presentation model for google and other search engines. Google likes seeing quality content on the home page that rotates through on a regular basis. Having a decent sized excerpt, that gives the search engine enough to get a good read on what the article is about with a link to the rest of a high quality, on topic writeup on the subject is exactly what the search engine needs. So well written articles in a competently setup WordPress blog tend to rank quite well. The pinging capabilities of WP helps a bit too, though that's not something that only blogs do

.
The problem (IMO) is that by having topic areas in your article system that overlap the topic areas as your forums, which is obviously how it's generally (and quite correctly) implemented, you split the ranking of your content areas between the forums and the article system. A forum thread can rank just as well for a given topic, the problem (again IMO) is the default forum homepage setup of LOTS of links with very little changing text associated with them (VERY short excerpts). So forums do great at longtail SEO (assuming they are otherwise well setup and google spider friendly), but
tend to not rank as well for the site wide core keywords that quality articles can deliver.
BUT if you can build your home page to present carefully selected member threads as well as threads that you have specifically written (essentially articles posted in the forums) in the same basic format as a "Blog" or "CMS" based site, you reap the same benefits. Instead of directing the Search engines into articles that are partitioned off and frequently have very few comments, you direct them to the same exact content, but it's likely to have much more commentary (in many cases) as well as being surrounded by other threads about the same general topic.
Put simply:
If your site is about cars, and you have a forum for each manufacturer, my contention is that it reduces your chance to engage new visitors (who may not even see that there IS a forum, or have a clue what a "forum" is) to have articles about Ford in one system and member discussions about Ford in a completely separate part of the site. Search engines will also see the site as two separate systems, and generally weight those two separate systems independantly (just as they do with subdomains).
If you're a blog, you should be a blog. If you're a forum, you should be first and foremost a forum... But that doesn't mean you can't leverage the lessons learned from blog design to help better establish your site as an "authority" site for your niche!
