holy cow. that means i am probably using a very overpowered server for my own forum and still it stays pretty much tensed throughout the day. the forum listed below is running on a dual quad core with 8GB RAM. though there are some other sites running on it. i have rarely seen server load below 1 i guess the server is not really optimized properly!
I have a vBulletin instance running 1.6m posts, 1,000 concurrent users.
As soon as Xenforo has the ability to have hacks through plugins and is integrated with enough caching (memcache) and search optimisations (sphinx) I will jump.
That said... sphinx is not as much a requirement as plugin architecture and caching. I can buy bigger boxes if need be, but dropping features would really risk a revolt amongst any long standing community.