Per the comment here [1] and the linked twitter convo, I'll retest S3 with Amazon Linux soon. These tests used Ubuntu 14.04 on all providers, and did use HVM. My understanding is that this will possibly increase the network throughput of the VM, but the benchmarks stayed below the VM's capacity (which was the reason I included the charts of VM throughput).

[1] https://news.ycombinator.com/item?id=10846497