Yeah, not really sure ... Google may have settled with the splash page now, changing it will conflict with the old set up and create a duplicate page - if you left the old page up.
But I was always taught that the index is most important for SEO, maybe if you just changed it a little ? Maybe add some RSS feeds on the (index) page - like new posts, new blog posts and anything else you could think of. What I am getting at is more links to the actual forum and posts. Like the news feeds you have on the portal page (but your own feeds).
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