<p>In my current project we are testing our ASP.NET GUI using <a href="http://watin.sourceforge.net/" rel="nofollow noreferrer">WatiN</a> and <a href="http://www.mbunit.com/" rel="nofollow noreferrer">Mbunit</a>.</p>

<p>When I was writing the tests I realized that it would be great if we also could use all of these for stresstesting. Currently we are using <a href="http://grinder.sourceforge.net/" rel="nofollow noreferrer">Grinder</a> to stresstest but then we have to script our cases all over again which for many reasons isent that good.</p>

<p>I have been trying to find a tool that can use my existing tests to create load on the site and record stats, but so far i have found noting. Is there such a tool or is there an easy way to create one?</p>

We have issues on our build server when running WatiN tests as it often throws timeouts trying to access the Internet Explorer COM component. It seems to hang randomly while waiting for the total page to load.
Given this, I would not recommend it for stress testing as the results will be inaccurate and the tests are likely to be slow.
I would recommend
JMeter
for making threaded calls to the HTTP requests that your GUI is making