AWS performance of RDS with provisioned IOPS vs EC2
I have done little testing on EC2 instances, but I have done extensive bench-marking of Oracle RDS instances with and without provisioned IOPS. 

In my experience the provisioned IOPS deliver close to the advertised performance. Eg. if you provision 1000 IOPS, you will likely peak close to a thousand IOPS with a response time of less than a millisecond, depending on queue depth.

If your workload is I/O-bound then provisioning IOPS may lead to a significant increase in performance. 

Note that IOPS provisioning is also available on EBS backed EC2 instances, presumably with comparable performance. 

When comparing RDS to EC2 I suppose it mostly comes down to how much control you are willing to give up for the convenience of using RDS over self-managed databases.