The second bullet I can answer. Mansystems has a tool for performance testing Mendix. Take a look at the website here: http://www.mansystems.com/application-performance-management/
The maker is an Mendix MVP and an active community member.
While I can't say it's ever been done, HP Load Runner says it supports AJAX applications here . Since a running Mendix application is simply HTML and AJAX, it seems to me that load testing should be possible.
If you try it, please let us know.
The issue with load test scripts is always parametrization. If you record a session and play it back with 100 users you need each play back to use it's own session identifiers, it's own user and password and most likely in the script at some places you need the output of a step to be the input of the next step.
With Scapa Technologies we have worked on automatically determining parameters for GUIDs.
I know the Mendix home grown solution for their professional services also automatically determines parameters.
Maybe a bit late, but I could not run the performance testing tool from Mansystems on Mendix 7.4 (so thinking from Mendix 7 release)
I did try using HP Load Runner (now bought by Micro Focus) with the application running on Mendix 7.4 and while I am no expert with performance testing, I was able to create scenarios, manage the load and get the analysis to a certain extent. Still at the trial and error phase...
You can use Jmeter to record all requests for a load test script. Then store the parameters in variables that can be used in the LoadTest. It takes quite some work for the intial set up, but it gives very detailed reports afterwards, be sure to run the load test in java.