user load

What exactly do you want to test?
The “users” mentioned in your title refers to “authors” or “visitors”?

Based on earlier posts around here, I’d say in the case of WP sites with huge traffic – the determining factor seems to be the server, not WP itself.
Check out some “famous” blogs:
http://codex.ww.wp.xz.cn/User:Matt/Famous_Blogs

Also, threads like this:
http://ww.wp.xz.cn/support/topic/70303?replies=49