I'm very interested in ECS's startup latency characteristics. Do you know offhand about how long it takes to provision a new container, and if this is subject to wide fluctuation?
I'd recommend setting up a private registry within EC2 though, otherwise you'll have a fairly significant delay while it pulls the image from wherever and incur bandwidth charges for doing so. You'll still have some delay while it pulls the image from your registry running on EC2 but not as significant if it were externally hosted.