The site injection for growth doesn't even make sense on paper. It doesn't matter where the GH is pinned, the IGF-1 is what causes the growth, and that conversion from GH to IGF-1 needs to happen in the liver, meaning you cna inject it anywhere and the growth benefits still need to come after that GH is carried to the liver to be converted to IGF-1. That stated, site injections of GH for fat loss are a real thing, as I've learned accidentally by not moving my GH shot locations around enough and getting small divots (from lipolysis) in areas that were pinned too frequently. Whether or not site injections of IGF-1 are helpful for growth is debatable but also not terribly likely in any measurable amount, else we would have heard about it a long long time ago.
The X factor here is also genetics. Why do we have some bodyparts that are bigger / more vascular / more responsive to training than others? There's a multitude of reasons, but one is that IGF-1 receptors are not distributed evenly throughout the musculature. So even if you think you are hacking the body with GH / IGF-1 injection timing / site pinning, you likely are not. Just take the GH, train hard, eat properly, and recover. The rest is pre-ordained genetically.