Once again Phil we agree. I too utilize the Mix function for the distribution of cold AC air. I also do not see how the air dam pictured above does anything but block the flow and increase resistance but happily my 2011 E-350 chassis has none of the heat issues previously discussed.
Usually by the time we reach the CG, check in and reach our site the engine has been operating at very low RPM with a fair amount of idle, for a while and most of the engine heat is gone. Never felt the need to prop open the hood but I'm sure that would enhance heat dissipation.
:C