Migrating Drupal 6 to pressflow?
I have a client who takes deliveries in the form of the sites/ folder and a database backup, and installs them in Pressflow.  As long as the Pressflow version isn't older than the Drupal version, we have never had problems.

There is no real migration.  Just replace the files, run update.php, and move on.  Two minor gotchas, though.

Pressflow error reporting is different, and a lot (and I mean a lot) of E_NOTICE will end up in the dblog.  You may want to adust error_reporting to exclude these.

Pressflow is API compatible with Drupal, but I have run into situations with anonymous sessions.  Occasionally, I need to manually call drupal_start_session, which is in Pressflow 6, from a custom module before I start doing stuff with $_SESSION.

Personally, I would give Zend Server CE a shot, too.  The Pressflow 6 site that I am wrapping up really flies in this environment.