"I must say we are currently in the process to moving to a master - slave architecture. We are having too much trouble with the master - master setup."
This is valuable information. We still might consider MMM (mysql master-master)-setup, but with a loadbalancer in between Drupal and MySQL-servers, which means that to Drupal it will seem like a single db. On top of that, there might be a slave for some selects, and one dedicated mysql server for a few very busy custom tables.
"The writeup is written for Drupal 6. But after reading your comment, I consider writing a second blogpost with contra's about master - master and ... most important ... our idea of the new setup."
I'd very much like to see that article also :)
"We are also considering the use of a common NAS drive where all files are stored. So we do have a single-point-of-failure on that one, but ... the chances the NAS falls down are so little ... It's better to keep the files directory clean"
At the moment I'd consider a NAS RAID-setup reliable enough for /files. Most of those files will be cached in Varnish anyway.