I have no experience with the Diamond blend you mention, but many swear by it. In any event, I suggest when trying any new food, buy a small bag and blend with your current food over the period of 5 or more days. If all goes well with the transition, then feed 100% of the new food for a month or two and gauge results (desire to eat, performance, weight gain or lack thereof, stools, etc..).
Bill is right about Purina. They pour money into the performance dog circuits. All of us that run in those circles, appreciate their support.
I ran the gamut on Pro Plan for my dog, and wanted it to work, but by age 2 I went elsewhere - to Orijen. Do what is best for your dog.