"The EC2 ephemeral disks normally clock in at 6-7ms latency, that's >13x slower than dedicated disks."
Um. Are you comparing hard drives to SSDs? Rotational latency for a 15k drive is a couple of milliseconds. Seek time for server drives varies from 3-10ms.
EC2 disks are slow, but there's no way they're 13 fold slower than your average server drives. And 6-7ms is just about on par with commodity hardware.
That's rather optimistic.
The EC2 ephemeral disks normally clock in at 6-7ms latency, that's >13x slower than dedicated disks.
EBS clocks in at 70-200ms latency, that's >5x slower than a dedicated SAN.
And that is under optimal conditions. In reality the I/O performance on EC2 frequently degrades by orders of magnitude for long periods of time.