I'm not sure that the article moved to bare iron. The author could have just set up postgres on a regular ec2 instance.

Yes, we did move to a regular EC2 instance for our cloud service. Where we install databases on premises for our customers it can be bare iron, but it can also be a similar virtualized setup.

Does that bring performance or cost savings? What is the motivation and calculus for making that transition?

It can bring a cost savings, but the savings is very slight if you're comparing apples-to-apples. For example, if you're using RDS in a multi-AZ replication set up with periodic S3 backups you have to take into account running each of the replica instances and the S3 costs. What we found was that RDS was treated like a black box to our automation (which was a good thing for a long time) and so we were less attentive to being over-provisioned and actually testing failover situations than we should have been.

30,000 iops is less then single crappy consumer grade SSD 6TB is fairly low limit largest instance has 244Gb RAM So improved performance, running larger instances, lower cost