I migrated a (after pruning) 20k users bbPress site to Discourse, and I can at least assure you that:
- SEO did not take a hit. (We’re still using our WordPress site as the landing page & blog. Discourse for comments).
- User engagement definitely seemed to increase (we didn’t collect enough data from bbPress to make a stronger case I’m afraid)
- There was remarkably little resistance to the change, in no small part “thanks” to many issues we had been experiencing with bbPress due to a nasty case of extensionitis. (but be sure to give them an early heads up, and let them be as much part of the process as possible).
I’m curious about your SSO situation. Are you using WordPress for anything that actually requires login? A “clean split” between WordPress and Discourse is greatly preferable. For us, it didn’t make any sense to be funnelling user logins through a WordPress site that could just as well be a collection of static pages (which it might just be, in the future). Discourse is our community, so it might as well be our primary logins database.
You can see the same thing being done here on Discourse.org.