i've thougt about the same wen building my undertray. theres a series of oval holes through the top and bottom of the radiator support beam... the top and bottom ones are not aligned, most likely for strength reasons. the way they look i'd say they're mainly for lighting the beam. the interesting bit is they give way to the area in front of the rad so one would assume they they decrease the pressure in front of the rad by allowing air out again, especially since they're directly behind the airdam so there should be a little lower pressure there . on my final undertray wich was also bigger i just closed them off.
the new tray did better then the original one so i left things as they are.
i've noticed cooling temps are a fraction lower with the tray in place, but i'm not sure if it's because the holes are closed off
heres a pic with the prototype tray with a cutout trough wich you can see the holes
