[GMCPaul]

They made the cowl vent doors easily removable so that debris could easily be removed. If you open the drain further all the debris will now settle between the fender and cab resulting in faster rusting from the inside out in those areas. If you park outside expect to clean out the cowl vents multiple times a year, if parked mainly indoors once a year should suffice. If you open it up to let the debris out of cowl expect faster rusting of rear of fender, and forward side and underside front portion of the cab. So in my opinion you'll just be trading a area you can clean out to a area you can not clean out nearly as easily.
Just my opinion.
Paul Sr @ GMCPauls