We just ran a 300 user stress test with Linux and moodle 1.7.1 and had no problems. In fact, we could have handled 1000 simultaneous users on the same single server. So roles is not making any issues. I had a similar breakdowns before with OS X as a server. Apache and MySQL configurations need changes if you stick with OS X--but I don't know what--just try comparing with our settings over in our report.