SEO is tricky with UGC, but I think there is a lot that can be improved in the software. I think SEO experts would recommend that content is hugely important, but if you can't do much to change it (without upsetting your users) then we have to look at what is practical and I think there is more that can be done with the software.
I installed XF2SEO because I had a few needs. It’s all about titles and descriptions not being implicit. My site is about Warcraft 3, but every node title doesn’t contain that name as it would bother a human but would be ideal for a search bot.
I have a Node called “Maps” where people can download maps for the game. Everyone knows it’s for Warcraft 3 but when people search for “Warcraft 3 maps” they won’t find my site as well as it should. So this add on allowed me to add all the implicit titles and keywords without spamming the forum list. I could also add words such as “multiplayer, campaigns” and so on. It makes a lot of sense for the important entry points to your site.
I made my own front page add on which has a particularly long title to catch anyone searching for the site. You don’t want to make too long and spammy “Site title” as combined with a thread title it would become far too long. So that made a lot of sense.
Those are the things that I have inplemented search wise so far. I hope it gives a good sense of what SEO actually is. I guess it can be summed up to not leaving anything to be implicit.
I would say both. But it depends. Many people have a forum for their product support. The product itself is the reason people visit. The forum is a secondary part of their site. A service.
My forum however IS the product and IS the site. In that case I would say definitely both. But the XF2SEO add on does a good job. But you can’t use it unless you know a thing or two about SEO. The things I eluded to earlier.