After the Part I of the blog was posted, @ericlaw shared on Fiddler Google group details on the alpha. He explained that the main performance benefits are directed to Windows Vista+. My Fiddler scalability test in the blog was performed on Windows Server 2003. Obviously, I was curious to find out what will be the Fiddler scalability boost on Windows 7. So I ran the same load test on Windows 7 and posted the results on the same Fiddler group thread. 
In a nutshell, the Fiddler alpha was 4-7x (!) faster than the current version (and so was StresStimulus).
While the alpha is making its way to prod, the last StresStimulus version materially addressed performance issued on Windows Vista+. It adds sessions to Fiddler after the load test completion, which offloads test engine and allows much greater number of virtual users (up to 10x). Next version, which is coming later this week, will allow adding to Fiddler sessions generated by one user and save time on loading sessions from other users. While additionally saving time on load testing, one user sessions still allows to analyze waterfall diagram throughout the entire load test.
In the future blogs I will compare StresStimulus scalability with some of the leading load testing tools.