since I just been luring around a bit and searched through MS Research papers... there is an interesting project called AJAX View. The idea is to observe and measure the behaviour of AJAX applications in the wild.
To do so a proxy is installed between web server output and what is actually delivered to the client which injects additional code into the web page. While this certainly will not boost the download experience (see one of my last blog posts) it will certainly help too measure your codes performance on the client...in the wild!!!
See here for details http://research.microsoft.com/displayArticle.aspx?id=1712