[kmike]

Apart from using Sphinx to search for the similar threads, you can also use it to generate the post excerpts with search keywords highlighted when in the "Show search results as posts" mode.

Our stats: almost 14 mln posts, 1.1 mln threads, 300k users, vB 3.8.
We're using our own Sphinx implementation since it predates the hack in this thread.

We got rid of the obscure search and sort modes though (such as sorting by the number of views or replies), and there was not a single complaint from our members. I don't think you should focus too much on 100% compliance with the default search. Having too many document attributes will inflate the index size, resulting in more I/O and more sluggish performance.
If you are worried about the need to edit the default search form template, you could always clone it, make the necessary changes and ship it with the product.