If you have to install Siebel v8.1.1 on IBM Power platform don’t hesitate to read this useful cookbook build by ORACLE/IBM team member.
Oracle recently completed an extensive performance and scalability benchmark exercise for Oracle Communications Billing and Revenue Management 7.3.1 that demonstrated its ability to support 100 million subscribers with IBM Power Systems.
The only EBS R12 official benchmarks published are on IBM Power. Last Summer 2008 Oracle with IBM conducted a benchmark in the Oracle/IBM Joint solution Center in Montpellier (France). This benchmark was Telco oriented.