they don't stop stuff from spraying back, they stop stuff from spraying up onto your side body panels. I've kicked rocks that have broken peoples windshields even with my PDP flaps installed, but I keep the rock chips off my paint!
and just to update, I installed my PDP flaps with factory wheels/tires in the OP. I've since gone Methods with 0mm offset which pushes each side out a bit more, I was worried at first my mudflaps wouldn't protect as much, but they still do a perfect job. Haven't noticed any difference at all from the increased track width as far as getting stuff flung on the paint.