Saurabh Gupta, our performance Program Manager, published a white paper a few months ago. It provides some performance comparisons between WCF and other existing .NET distributed computing technologies:
http://msdn2.microsoft.com/en-us/library/bb310550.aspx
The technologies compared are:
Different transports and settings are used in the comparisons, which show how WCF performs in different dimensions.
Marc Mezquita, our performance tester, published the white paper for Windows Workflow performance a year ago at:
http://msdn2.microsoft.com/en-us/library/Aa973808.aspx