Would a topic related forum added to my site bring any benefit for SERPs, or is it a lot of effort for little return?
I don't see how it could hurt... which a seo mod you can get all your posts indexed in search engines.
Don't think that is quite worth it at the moment for you. It wont hurt at all, but with only 2k hits a month you would have to do some extra advertising for your forum. If you have spare time, why not