I think it slightly slows down growth. I'm after longevity not fast growth. When I fed them the game bird I had some with leg problems. Mine are heritage. I raised each batch after with chick starter, because I had some and didn't want to go get the game bird. They grew find, were energetic and are still with me. I did grow another batch on game bird since, but really didn't notice any real difference.
My thought were that the wild turkeys around here are raising the poults on range and the are fine. So this is what I have decided to do. I have never lost a poult and they are always healthy. Last year's are about 3/4 the size of the adults, they aren't quite a year old year, so I don't think in the end the growth is compromised in any way.