Migrating to Pressflow
Pressflow 7 doesn't seem to contain any relevant performance improvements at this time.

Below is a diff from 7.10 to the pressflow/master branch on github, where pressflow is now maintained. (https://github.com/pressflow/7)

$ git diff -R pressflow/master 
diff --git b/includes/bootstrap.inc a/includes/bootstrap.inc
index d63a59b..e5e7ab7 100644
--- b/includes/bootstrap.inc
+++ a/includes/bootstrap.inc
@@ -728,6 +728,22 @@ function drupal_settings_initialize() {
   }
   $is_https = isset($_SERVER['HTTPS']) &amp;&amp; strtolower($_SERVER['HTTPS']) == 'on';

+  // Load environmental config, if present.
+  if (isset($_SERVER['PRESSFLOW_SETTINGS'])) {
+    $pressflow_settings = json_decode($_SERVER['PRESSFLOW_SETTINGS'], TRUE);
+    foreach ($pressflow_settings as $key =&gt; $value) {
+      // One level of depth should be enough for $conf and $database.
+      if ($key == 'conf') {
+        foreach($value as $conf_key =&gt; $conf_value) {
+          $conf[$conf_key] = $conf_value;
+        }
+      }
+      else {
+        $$key = $value;
+      }
+    }
+  }
+
   if (isset($base_url)) {
     // Parse fixed base URL from settings.php.
     $parts = parse_url($base_url);
@@ -2329,6 +2345,17 @@ function _drupal_bootstrap_database() {
 function _drupal_bootstrap_variables() {
   global $conf;

+  // Pressflow Smart Start
+  if (!empty($GLOBALS['databases']) &amp;&amp; variable_get('pressflow_smart_start', FALSE)) {
+    try {
+      $result = db_query('SELECT s.name FROM {system} s WHERE s.name = :name', array(':name' =&gt; 'system'));
+    } catch (Exception $e) {
+      // Redirect to the installer if an essential table is missing.
+      include_once DRUPAL_ROOT . '/includes/install.inc';
+      install_goto('install.php');
+    }
+  }
+
   // Initialize the lock system.
   require_once DRUPAL_ROOT . '/' . variable_get('lock_inc', 'includes/lock.inc');
   lock_initialize();
diff --git b/sites/default/default.settings.php a/sites/default/default.settings.php
index f8894aa..08fc81b 100644
--- b/sites/default/default.settings.php
+++ a/sites/default/default.settings.php
@@ -487,3 +487,13 @@ $conf['404_fast_html'] = '&lt;html xmlns="http://www.w3.org/1999/xhtml"&gt;&lt;head&gt;&lt;titl
  * Remove the leading hash signs to disable.
  */
 # $conf['allow_authorize_operations'] = FALSE;
+
+/**
+ * Smart start:
+ *
+ * If you would prefer to be redirected to the installation system when a
+ * valid settings.php file is present but no tables are installed, remove
+ * the leading hash sign below.
+ */
+# $conf['pressflow_smart_start'] = TRUE;


As this shows, there are only two actual changes, one allows to set $conf values from the environment and the other is a safety check when Drupal isn't installed properly. Both are probably only really relevant for their Pantheon hosting environment.

So, while the interview posted by @wiim is nice, it's two years old and nothing has really happened. So, you can IMHO save yourself the hassle of switching for now (of course, this might change).