Also, my preference would then be to have the PSU completely isolated from the main chamber, and have hard drives in the front, so the PSU pulls outside air over the hard drives, then into itself, then exhausts it.
Is this any different from the bottom-mount styles of cases like the P180/182?
tehcrazybob has a good point about the PCB size, but more importantly when you have an "exhaust" fan, or negative airflow, without an airguide, you will be exhausting air from everywhere in the vicinity and it will be the airflow with the least resistance (not the air around the heatsinks and components). With an intake fan or positive airflow you can direct the air where you want it, knowing that as long as there is ample venting, it will be exhausted properly. It's kind of a weird dichotomy with the negative airflow that the ATX standard works with, but...there it is.