Hi @djay well, yes we do use a multi instance + multiple different CMS based Intranet at and Extranet at LMU backed by Solr. @reinhardt already pointed to the corresponding repositories.
We did not choose Solr to save RAM or make indexing quicker, but to search accross multiple sources and to have a better support for german. Solr is normally used with a schema, elasticsearch instead schemaless.
Solr is extremely configurable to adapt it to specific use cases.
But it is with all search engines you need someone to take care of the index.
Solr basic template for Plone is better than the Plone search out of the box, but could be improved a lot. Include data from any analytic could help to improve the index, but that is probably more complex than just boost them by those terms / sites.
For each larger setup I would always use an external search engine, today.