I did a load test for an ASP.NET MVC application. It was basically for 2 different pages which are doing the same thing but one was synchronous and the other was asynchronous. The result was shocking. While average response time for the synchronous one is about 11.2 seconds, it is 3.65 for asynchronous one. I think the difference is pretty compelling and overwhelming.
I have written a blog post on that as well:
Task-base Asynchronous Programming in C# 5.0 and ASP.NET MVC Web Applications
That'd be great to have a peer review on that blog post.