Are you creating these review app clusters via CI? If so you can update your CI’s creation flow to enable Postgis at cluster creation time by using fly mpg create --enable-postgis-support.
This will create the cluster with PostGis enabled out of the box, and doesn’t require using the UI to enable it. Does that work for your use case?
Granting full superuser permissions to all users isn’t something we’re planning at this time, though with MPG v2 we might be able to offer better access to features that otherwise require SU perms.