You are right on the apache modules.
For load testing: difficult, as you can only hit a certain page, not simulate clicking though the pages. But for example
loadimpact.com/index.php can get you started online with simulated loads. Keep in mind that 50 users hitting your server at the same time is different than having 50 users take a survey and click through pages at varying speeds. There are other tools, like
httpd.apache.org/docs/2.0/programs/ab.html but they need some experience to know how to use it right.
So I would say you can have quite a lot of users taking a survey, especially if you are not too heavy on conditions. The array filters seem to cause quite some load on the server somehow.