I've been running the CS/GT/Boss lower valance and splitter for over a year now on my 2011. I've been fortunate in not obtaining any radiator/AC condensor damage. I have found that for around 50 bucks, I can buy a black grill piece to install in the lower valance. I have not done this, yet.
However, I should also mention that the roads and highways here in Florida
are pretty well maintained; much more so then those who perhaps have to drive in pothole stricken highways of the north where salt, snow, and freezing temps play havic on the roads.
if you drive in these types of road conditions, then protecting the front of your prized Mustang and its radiator/AC condensor would be a great idea, and installing a grille in the lower front valance would be good insurance.