I'm going to watch this carefully. We probably can't afford a server upgrade for awhile, and we're maxed out on our server at 4GB right now. If ES is going to exhaust our limited memory, we may have to pay someone to do an update to the Sphinx add-on instead. Or just tough it out and stick with vB 3.7 (which I have grown to really dislike lately).
I'm on vB 3.8. When I do eventually move over to XF, I'm going to try running ElasticSearch on a cheap Amazon EC2 instance so it won't affect the memory usage of my primary server. The "Small", standard instances (1.7GB RAM, 160GB HD) can be reserved for $97 per year.