The Product Assurance team have released an updated copy of the OBIEE Tuning Whitepaper.
You can find it on the PA blog https://blogs.oracle.com/pa/entry/test or via Support note OBIEE 11g Infrastructure Performance Tuning Guide (Doc ID 1333049.1) https://support.us.oracle.com/oip/faces/secure/km/DocumentDisplay.jspx?id=1333049.1&recomm=Y
This new revised document contains following useful tuning items:
1. New improved HTTP Server caching algorithm.
2. Oracle iPlanet Web Server tuning parameters.
3. New tuning parameters settings / values for OPIS/OBIS components.