with negative pressure, case will be pulling in air from every hole/gap - filtered or not filtered. When in Positive (more intake fans) airflow is via that direct airflow path. In theory Negative gets your better temps , but dust dust dust.
if your GPU is aircooled then rear exit fan will help , but if underwater and you want the max lowest water temp, you'd have bottom and front as intake with rear as exhaust- but doesn't always work the best