We've just expanded the configuration options for Postgres databases using network-attached storage.
On the "Storage" tab of your database's Clusters page there are now options to configure the provisioned input/output operations per second (IOPS) and bandwidth.
The maximum amount of IOPS and bandwidth that can be provisioned is based on the cluster size of your database and minimum disk size.
See Cluster Storage for more information on configuring network-attached storage on Postgres clusters.