Hey @Sc7565
what did you mean exactly with ES Clients, did you mean coordinator nodes?
I did the experiment by taking the same node type in my cluster, and adding a new member with ST1 EBS type to the cluster, but it seems from the monitoring that it performs poorly compared to the GP2 volumes, the new node IOPS were consistent near 100% and CPU usage almost 100% most of the time.
I'm considering now moving to ephermal storage instances to save the volume cost, and by purchasing saving plans to the ephermal storage instances we can reach a fair amount of costs savings.