We have it running in production and its very fast; granted the size of our user base is small.
www.diecastmodelaircraft.com
What we have done:
xcache - using that for PHP caching and we have turned on vb variable caching via xcache as well.
No-Cache - in vb settings, make sure "Add No-Cache HTTP Headers" is set to no
Gzip- Ensured gzip compression was on in both vb and for our server.
Mysql - We tuned our mysql system over the past few years. use mysqltuner.pl to assist.
Mod_pagespeed - Experimenting with Googles' mod_pagespeed - turned off right now; but will try this again.