Amazon: How do I know if I need Provisioned IOPS?

Do non-"PIOPS Optimized" instances still benefit from Provisioned IOPS?

Yes. You benefit from the PIOPS volumes being SSDs under the hood. Optimized instances have dedicated network to the EBS servers, so they get consistent performance. Non-optimized instances interact with EBS along with the rest of their network traffic, so heavy bandwidth use on the physical host can slow you down.

http://aws.typepad.com/aws/2012/08/fast-forward-provisioned-iops-ebs.html

As a point of reference, a standard EBS volume will generally provide about 100 IOPS on average, with the ability to burst to hundreds of IOPS on a best-effort basis.

With this in mind, I suspect you'll see performance improvements from PIOPS on the RDS, as you're averaging 50-100 writes and 200-400 reads on the RDS volume - you're likely in that best-effort burst state most of the time.

Your web server is doing zero IOPS from the charts, so I'd guess everything's already living happily in RAM and not hitting the disk. Do check that it doesn't have a secondary volume you should be looking at instead, though.