I'm using Xenforo for my company's internal board which is expanding to include external users as well. That said, it will not be a large board for a long long time - I'm guessing under 1000 hits a day, if that. The cost for enhanced search is really not that big of a deal to add greater search capability, control, or speedier results.
However, is there any benefit for smaller boards using Elasticsearch over the fulltext MySQL capability?
I think it's clear it's more performant for larger sites than MySQL fulltext search. Highly customizable, purpose-built, memory-oriented (if you let it be), so it's great for large boards - I got that message loud and clear. What I'm wondering is if it's also valuable for smaller boards - or if the value is purely performance for a large data set.
I currently use it on my largest board - but plan to slowly add it to the smaller ones as funds allow; specifically because of the stemming and no min. word lenght (which I've manually set to 3 in MySQL anyway).
If you can easily afford it, have the server resources to run it, and like the idea of the additional features - then there's no real reason why you shouldn't use it for a board of any size.