Earlier quoted context omitted.
You might also take a look at the search functionality in Riak. I've run both Solr and ES, the latter at significant scale, and I'm leaning more towards Riak going forward. The difference is mainly convenience, so not a reason to switch off something that's working already.
Hadn't considered Riak, but I can see that it has some full-text search capabilities. Any idea about its features and how it compares in performance, as a raw search index?
Edit: Apparently my Riak knowledge is dated now anyway. It looks like I have some research to do myself, but it's pretty exciting stuff.