Hi
Since you are using Server Proxies, I will also recommend you to go though AAE queue prioritization if you are using SAP PI 7.1 or SAP PI 7.3x.
PI 7.31 AAE Message Prioritization: http://help.sap.com/saphelp_nw73/helpdata/en/48/b2e00f6b156ff4e10000000a42189b/content.htm
PI 7.1x AAE queue prorotzation: https://sites.google.com/site/ruslimchang/message-prioritization-in-advanced-adapter-engine
Regards,
Nipun