Yes, our defaults are way better than an out-of-the-box Postgres. That said, we are acutely aware that significant further tuning specific to a particular database's workload can deliver even better performance. We are investigating the best way to deliver this to a broad audience now. Do you have any suggestions?
Amazon RDS uses Elastic Block Store (EBS) as its disk. EBS is extremely slow relative to the SSD drives used at DigitalOcean. Worse, unless you buy very expensive provisioned IOPS, you are competing for network bandwidth with all other EC2 customers in the physical network segment where your RDS database is hosted every time you do a disk access. This means your database will randomly become very slow at peak load times, with no visibility and no way to control it other than spending a whole lot of money on provisioned IOPS.
We've also gotten good feedback from a number of customers who simply do not want to deal with the complexity of AWS. Our API is far simpler; we don't have security groups and so on. If you actually need all that complexity, that's fine, but many people don't.
Finally, RDS does not support read replicas for Postgres (at all); you are limited to one database server. We are currently setting up replicas manually as requested, and will be building them into the user interface in the near future.