[SR-Users] Kamailio performance testing

Hi I am trying to test performance of kamailio server. I have made some tests on old hardware, that was ok. I found that it can handle around 450 calls per second with authorization. CPU utilisation was up to 100%. That testing was OK, because server's CPU utilization was increasing. Now I want to do some test with new hardware and do some comparisons of those two hardware configurations. I have made some test but i found that it can handle up to 540 calls per second with authorization. But CPU utilisation was only around 8%. When I increased call rate server responded with 404 Not Here and 500 I am terribly busy messages. I think that it is because of that it is not capable of finding a user location that fast. Can i do something to increase the performacne of this server? I am using SIPp testing tool for generating calls. I have registered users manualy with kamctl ul add sipp sip:sipp@192.168.1.1:5060 command. I was using this tutorial: http://www.troubleshootingwiki.org/OpenSER#Stress_Test_The_SIP_Signaling. And also i made my own scenarios. Any suggestion would be usefull. Thanks.