ArmA 2 is pretty piggy on resources and has a million poorly documented performance options that can be set on its command-line and configuration files.
If you didn't see an improvement from adding a core, it sounds like you are having a problem with the server not threading very well. Have you tried different -cpuCount=X options on the server command line? Are you manually setting affinity, or letting the OS do it?
This page describes the -cpuCount option a little: http://community.bistudio.com/wiki/ArmA ... Parameters