You can try something like what they have at http://loadimpact.com/ or write your own.
If you have the opportunity to deploy a few simple scripts that simply secured a bunch of pages, you could easily write something yourself.
What you need to remember:
Your forums will be database driven. If your test test is just reading spam on one page, the request (s) will be cached and will run faster than in a large forum where users often execute more requests than the request cache can store. Memcached presents the same problem for your load test.
Make sure you check messages as well as views
, , cURL script (s). , ( -), ... , db , .
5 , 10, 50, 100, 1000... , . , , .
! , , -:)