I assure you that contention was not an issue on a restored backup. At that point PostgreSQL did not have the option to have read-only slaves. But for reporting, day old data was acceptable so that wasn't a problem.
It still doesn't solve the problem that you really want to tune differently for OLTP and data warehousing, and the server was tuned for OLTP.
I'll definitely agree that attempting to run OLTP and analytics queries on a single PostgreSQL server is not going to turn out well. I'm not sure if there's a way around it though, even in theory (e.g. MapReduce isn't very good for OLTP).
It still doesn't solve the problem that you really want to tune differently for OLTP and data warehousing, and the server was tuned for OLTP.