Do you recommend me to update to it instead (ie. would the MovieDB scraper be up-to-date in Jarvis-final?)
I do not know, we have to wait until the final Sam announces for all platforms OSMC version of KODI 16.0. I assume that all current addons are thus available then.
Also, do you know how can I update from RC3 to stable? (is this automatic via myosmc?)
I think Sam will distribute Kodi 16.0 Final determined within the next OSMC update.
Let us practice in patience