Thanks Dom, I missed that one in 2007. Good info though and hypothesis on why its happening.....not so much direction-of-car pressure as flow across the opening sucking air out.
My ducts are fed from a custom fabbed scoop under the fascia. Works great but makes the car one inch more likely to scrape. I was just interested in the logic behind the long-standing "truism".
Also the 105 mph thing makes it almost moot. You generally need brake cooling after slowing down, so most often you'll be going way less than 105 when you need the airflow. An exception is two big straights separated by a hairpin but that's not too common.
Thx for posting that old thread!