Ed Glas, Bill Barnett and Jiange Sun published a detailed write up of their experiences load testing VSOnline/TFS.  It’s a good read because they go into a lot of the reasons for the things they do and also talk about things they’ve tried that didn’t work.  They’ve now moved almost all of their load testing to the VS Online load testing service and had great results with it.

http://blogs.msdn.com/b/edglas/archive/2013/11/27/load-testing-visual-studio-online.aspx

Check it out,

Brian