In which processing step does the timeout occur? If it's on the outbound side, you may have to increase the processing timeout in either PI or SAP backend system, depending on what PI version (7.1 or 7.3) and interface are you using on backend side (RFC or proxy).
Have a look at the http://www.soapui.org/Data-Driven-Testing/loadtests.html for integrated load testing capabilites of SoapUI, they even have an own tool explained in detail here http://www.soapui.org/Getting-Started/load-testing.html.