in my view, sitemaps are an important piece for indexing any new site.
(recommeded reading: Learn about sitemaps - Search Console Help)
I think that it will help a lot to discover your new topics on a daily basis, and not only by Google, think also in Bing or the next search engine. Also, there is a reason why still you need (and google suggest) to add a sitemap in Webmaster tools, it helps to discover new links.
I’m doing a test right know:
One of my forums is online since early this week, many different new topics are there, and still is not indexed (even considering that I’ve notified this new site in Google Webmaster Tools and Google Analytics).
Now I’ve added a sitemap (using the code provided here).
My guess is that tomorrow morning, it will be indexed, just because I submitted the sitemap.
Will let you know!