As original poster allready said, using "ALTER SYSTEM FLUSH SHARED POOL" has no impact on database buffer cache whatsoever. There is no such command to flush buffers from buffer cache.

I can't verify the following right now, but I seem to remember to read somewhere that if you take a tablespace offline, all the buffers that belongs to segments from that tablespace are freed from buffer cache. So if you can afford it, take all the tablespaces that are involved in your query offline, put them back online and benchmark your query. However verify first that the above trick realy works, as I never tested it.