Well first hand experience with major diet changes with my 3 dogs ( now 4 but cant use the fourth as he's only been here a few months.)
I fed Pedigree, UNTIL I started with my pet food interests and research.
After learning BUNDLES of information, I switched them to Solid Gold Wolf King,
Their energy levels skyrocketed and lasted so much longer, their coats became very glossy and vibrant looking, their poops shrunk and little infections cleared up ( yeast in ears)
After that I switched to RAW and now... the energy levels are even higher, their stamina is amazing, doggie smell has deteriorated, teeth are white and clean as a whistle, shedding has reduced considerably ( even though its now spring here) and their muscle mass is amazing.
Food makes the world of a difference with dogs as it does with humans, sure some dogs live long healthy lives on Ol' Roy or Pedigree, but until you see what YOUR dog will look like and act like on an all around GREAT food. You'll notice other dogs that clearly arent on a high quality kibble.
I see hundreds of dogs at work on a daily basis, and its amazing how for the most part, I can pick out the ones fed lower end foods. How?? Cause they look my dogs used to.