I used PostgreSQL 8.4 for Pentaho OLAP (mondrian) for a while. It's work like i want. But all i've done are the combination of Application level, Database level and Hardware level.
Our application is CRM, Then we did ETL to new PostgreSQL server, and do OLAP there. The application and OLAP DBMS is totally difference configuration. In Hardware level with hardware optimization and postgresql.conf configuration. We can run OLAP query for our need.
I think you have to know your database behavior and what OLAP you need to query. Then you will know how to configure your OLAP and postgresql.conf and Hardware you need.
Benchmark tool is only a test. But you have to know all about pg_log and performance monitoring.
Wattana