These docs focus on the out of the box integration. Where Siebel holds the reins and just sends the BI Publiser server publishing requests. For this you use Integration Objects to generate the data. This support doc contains a white paper on performance testing of the IOs and provides some standard tests that you can compare your system to.
I have pulled the white paper out of support to save you some time (cos Im kind like that.) Of course the latest and greatest will be on http://support.oracle.com
The following support doc covers the tuning of said objects to handle larger data sets.
Improving the performance of Siebel BI Publisher Report Generation (Doc ID 1392449.1)
I have linked a converted PDF of the doc as of today, access http://support.oracle com and search via the Doc Id for the latest and greatest.
For completeness of the post, heres a link to post on the Siebel-BIP Business Service Integration.