RDS Postgres - Is lower provisioned IOPS better than higher baseline IOPS?

I think that is answered in the other thread you found, and I did add the link to a video where Grant discuss bursting (he talks about both T2 and GP2 as they were newish capabilities at the time). Thanks for giving me an excuse to talk about this some more...

The main thing I'll re-emphasize is that IO1 has a better SLA (both explicit and implicit) than GP2, which largely explains IO1's high cost. By way of (not necessarily accurate) explanation, the bursting concept, such as used in GP2, is one of over-provisioning resources based on statistical and historical usage patterns. So you mix different volumes on a set of resources (SSDs, servers, network adapters, etc.) that can support them going at baseline but not all going at burst. You do this based on both the volumes' historical activity and statistical assumptions about how likely actual IO will approach the resources' capacity. If the usage across multiple volumes changes from historical patterns, and you approach or exceed resource capacity, you consider migrating volumes to re-balance the load. During the hot periods, and any re-balancing, your volume can see increased latency, and potentially less than its targeted IOPS. This is all designed to keep the latency and IOPS within a range that most apps would never notice it, though if the app is super-sensitive to latency it can.

With IO1 (again, not claiming 100% accuracy in my explanation) volumes are placed on resources that can deliver all the provisioned IOPS of all the volumes they host. So one volume's performance doesn't suffer as the usage patterns from other volumes assigned to the same resources change. That doesn't mean there isn't some variability in performance, but it is within a much tighter SLA that takes into account all manor of occasional wonkiness found in any complex system.

ok. thank you!