No need to take the bumper off to remove the upper grille. It is held in with push clips and so all you need to do is tape off around the grille to protect the paint and pull straight out. That is how CDC did it when they did the promotional make over of my car in Mustang Alley during the Woodward Dream Cruise last August.
And yes, for the performance grille you have to cut the center section out
because the performance grille is completely open:
You can see the center is blocked off on the stock grille (at least for the MCA and I believe that the center horse on the stock grille also has the big block). You can also see the push pin type clips in this picture.