A few months ago I read: "The Impact of Virtualization on Network Performance of Amazon EC2 Data Center" (http://www.cs.rice.edu/~eugeneng/papers/INFOCOM10-ec2.pdf) which has some performance numbers (latency and throughput) for small and medium instances.

Unfortunately, I can't recall enough of the paper right now to give you the nickle-and-dime overview, but it has graphs you can look at!