The Microsoft Extreme Computing Group regularly publishes an extensive set of benchmarks on all aspects of Windows Azure performance at http://azurescope.cloudapp.net/Default.aspx. (Note that it is itself a Windows Azure application.)
We blogged about this before, but there have been significant updates since then.
These benchmarks are intended to assist you in architecting and deploying your applications and services on Azure. Included in the benchmark suite are tests of data throughput rates, response times, and capacity.
Each benchmark is run against a variety of test cases designed to reflect common use scenarios for Azure development.
To my knowledge no other cloud vendor publishes such a comprehensive set of benchmarks!