The IPB product page lists "MySQL 5.x not yet supported - news on MySQL 5.x support coming soon."

Does IPB 2.2 (including RC2) support MySQL 5.x ?

Yes.

That's a relief because my host only supports MySQL 5.x :)

and the MySQLi API Client for mysql

The company website hasn't been updated for 2.2 yet, since it hasn't been released finally.

and the MySQLi API Client for mysql

Whats that?

You can read more about what MySQLi is, here: http://ca.php.net/mysqli and some benchmarks mysql vs mysqli are available here: http://www.johnjawed.com/benchmarks/

Is anybody using MySQLi for running IPB and, if so, have you noticed any real-world performance improvements over standard MySQL ?

Is anybody using MySQLi for running IPB and, if so, have you noticed any real-world performance improvements over standard MySQL ?

If you have MySQLi installed into PHP, it will use it automatically. Chances are, most people are using it but didn't know it.