Dave_Hybrid - 4:51 pm on Mar 3, 2013 (gmt 0)
Tried exactly this with a forum, no index category's, thin topics etc.
Made zero difference.
The thing with panda and penguin, your site may have no issue, the sites that link to you could.
Many cases of panda demotions I've seen could simply be fixed with some better backlinks.
If you have a weak backlink profile, I'd look at that as well as cleaning up the site.