This is what I mean by fully testing the configuration; test all the operations that need to be done in the system, in the real amount they will be done in real life. Take the trace of a full hour (normal), and even a full hour where the load is higher than ususal. Then run that trace on all configurations, then figure out what configuration gave you the most bang for your buck.

This is not a fast process, but this is pretty much the only way to get all you can get out of a machine.