It would be interesting to run these tests on different instance sizes, specifically for data on the instance store. The larger the instance, the fewer neighbors you have to worry spending those precious IOPS.
As for SSD vs Magnetic EBS, I can't say that I'm surprised. I'd assume that EBS implements some sort of cache in between you and your actual disk on the other side of the network so that the writes can return even faster. Try doing this again with reads and I'd bet you'd get some interesting results.
Yes, I did pre-warm the volumes before using them.
And yes, there are several interesting workloads that I didn't test, including read only and read+write. It's potential material for another blog post.
As for SSD vs Magnetic EBS, I can't say that I'm surprised. I'd assume that EBS implements some sort of cache in between you and your actual disk on the other side of the network so that the writes can return even faster. Try doing this again with reads and I'd bet you'd get some interesting results.
Edit: Also, did you pre-warm your EBS volumes? http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-prewa...