We currently have ~1.2 million posts and while SQL search is slow, I don't think a lot of users rely on it since we've also enabled Google search as an option and their indexing is really good except for the newest stuff. On the other hand, we are working on an add-on that can search the database and come up with similar threads automatically.
My question: As a rule of thumb, will ES increase the amount of storage space required? Our current search index takes about 2.5GB.
* We will enable stemming, of course