Yes, I depend on WPEngine very much as the project is complex and I am more than happy that I can outsource things like performance, security and scalability.
As with WP_Query, I think, 8 fields will be enough for filtering in most cases. I think even 4-5 fields will be enough. So let me rephrase the question - things are going slow after 8 fields exactly or each field causes slowing down? I mean filtering 1 field is faster than filtering 2 OR filtering 1,2,3,4,5,6,7,8 fields require same time? In other words, does WP_Query have problems exactly after 8 fields (cause that's how it's configured right now) OR it's speed problem even when you are filtering 2 fields BUT it isn't noticeable?
WordPress search does not allow that ability to search across multiple sites, are you using another solution for that?
I am going to run WordPress Multisite, not single installation and while it doesn't offer such functionality by default, it's relatively easy to achieve that with custom coding (actually there are pretty good plugins out there which we are going to use). Few examples are WordPress.com search system which searches in millions of blogs and Edublogs.org which let's users get search results from almost 2 million sites (their sites of course, they are running Multisite).