Unfortunately, I think that all information about SEO benefits of tags, whether pro or con, will be purely anecdotal. I first became convinced of their benefit from a WordPress site I run on the same topic as my forum. I found that it (in particular the tag pages) did as well or better than the forum did for any keywords that were tagged, despite being a far newer site with far less PageRank and far less traffic.
Search engines seem to be using other signals more and more, but the link graph will always be a valuable source of information for them, and the more we can improve it (including with anchor text) the better we can do with SEO, particularly with long tail searches (where forums can thrive). Of course, it's also terrific for usability and helping people find great old threads related to their interests.
One of the core issue that tags solve is that most forums have some really amazing threads and quite a few weak ones. Unfortunately, once the amazing threads fall of the first page of threads for any subforum, they have almost no inlinks with any pagerank. By tagging our best threads, we can create a network of links between our best content. By adding a tag cloud to a high pagerank page, we can send pagerank to the entire network of "best threads."
Anchor text is another big benefit of tags.
I did a series of 4 or 5 in depth posts in a thread called "XF SEO tweaks." The following is the first post in that series. After that first post, the topic of the thread more or less turned to tagging, so if you start there and keep reading, you'll get a good overview:
XF SEO Tweaks
I will say that I've become less focused on curated lists of tags. Especially on a larger site, you want a big list of tags so that your users will bother to add the tags. It sounds like
@Alfa1 has gotten this working well on his big board.