I tried the CMS and the mod you are talking about.
I also heard of duplicate content having some kind of penalty.
I still use the CMS for articles, it is good for that but there are problems with comments and pictures and it isn't really worth the effort.
The integration of CMS and forums is better done manually.
By that I mean the thread that is created by the CMS is better posted in an invisible forum. I believe this is established practice.
What I do now is create the article (turn off comments) and then create a link for comments to another forum that is created by myself. It is much easier for comments to be added, it is more manageable and also negates the duplicate content and the attachment issues.