High performance DB Cluster with Drupal
Galera Replication for Drupal

Things you need to watch out for:


Caching
Sounds like you are using memcached, that good because drupal caching needs to be disabled because if one drupal node modifies the cache stored in the db and that change
replicates the other drupal nodes don't know about it creating conditions in which the drupal cache can serve stale content. Most people use memcached for caching to get around this.
Deadlocks
On websites that are high writes you can incur deadlocks when two drupal nodes try to update the same row at the same time. In this case you would want your app to retry the action but Drupal doesn't always do that. In the Galera settings you can set up some settings in my.cnf (wsrep_retry_autocommit=n) to enable retries at the Galera layer to prevent deadlocks being sent to drupal. Although for a website with low writes and mostly reads you may never encounter the issue.  


Other than perf testing a realistic workload I don't know of a formula for performance expectation.

For Galera I've used both MariaDB and Percona. Percona does have additional information it can expose for performance and troubleshooting. Not a lot of difference in how they use Galera replication. I've moved innodb databases between percona and maria and had no issues.

I've found a Percona whitepaper on galera performance and best practices for drupal. It seems to cover all the issues I've encountered plus some additional info: http://www.percona.com/files/presentations/percona-live/nyc-2012/PLNY12-galera-cluster-best-practices.pdf

At this time I would use Percona Galera or Maria Galera. I don't touch mysql from Oracle anymore.

hope that helps