It's not a throttle, it's the "physical" capacity of the "interface". If you're on an instance with gigabit connectivity and you're doing 1 Gb/s of EBS I/O, other network chatter will suffer, probably fairly dramatically. That's why the high-I/O instances have 10 gigabit connectivity, as I understand it.

Happy to be proven wrong but this is based on a year or so of experience dealing with EBS. You can't see the EBS traffic in your tools (at least that I've been able to find), which complicates things.

It's all kinds of different on VPC instances, therefore I suspect the network interface model -- and possibly EBS connectivity -- is different on those. So, who knows? I'd kill for Amazon to be more forthcoming here so I could understand the infrastructure running my fleet, but, I don't and they aren't.