The Extreme computing group has released a fairly comprehensive set of benchmarks  for almost all aspects of WA. They have also provided the source code to alleviate all doubts that may surface with the MSFT logo lurking around the top right of their homepage :) (Which also resides at a cloudapp.net url).

The code is simple and the tests comprehensive enough to hold as data points for customer interactions. Add to it the clean no nonsense Silverlight charts to render the benchmarks and you are set to sell.