Is there a disadvantage with including posts in sitemap?

I run a big board with 3.1+ million posts and 220 000+ threads.
My sitemap is currently not including posts just threads (a setting I chose when first installing vBSEO to get more relevant indexing)

So now I wonder what will happen if Turn indexing of posts on, isn't it good to have as many options as possible for google to hit?

My personal recommendation would be to keep posts out of your sitemap.

All of the content covered within the posts will be found within the thread pages themselves. Having only 220,000+ thread pages to crawl is far more efficient than having 3.1+ in addition to that for Googlebot.

Keeping the bots focused on the threads instead will ensure more complete indexing of your forum.

If the showpost links are still open to guests, they'll still also crawl those as well. HOWEVER, you should keep the sitemap directing them primarily at the threads.