Just some more anecdotal input for you. I have never had a dog that experienced pano.
Our childhood GSD was mostly fed table scraps with some dry kibble added. My first GSD and my sisters' GSDs (who were her pups) were also fed this way. We bought the dry kibble at the grocery store - but this was several years ago (back when IAMs was thought to be a top quality kibble and prior to special puppy formulas). They all were large GSDs, over the standard size, once full grown. I don't recall any of these dogs ever going through pano.
Dalton my old boy was fed a rather low quality kibble during the first years of his life and was never fed a puppy formula. His growth rate I would say was normal. He was on the larger size for a GSD, maturing at a healthy weight of 87-88#. I don't know about his siblings or parents but he never experienced pano.
Sundance comes from generations (mother & grandmother) of raw fed dogs and has always been fed a raw diet. Now his growth rate was shockingly fast to me. I remember being real worried about pano with him just because of how fast he grew. I was very careful about his diet and really tried to control his growth while being careful to assure he received the proper nutrition but nothing helped. He is just over 17 months of age and 87.5#. The vet says he is a healthy weight but could maybe gain a pound or 2.
I'd have to ask his breeder if his siblings or parents have ever had pano. He has never experienced pano.
Butch has been raw fed since 8 weeks of age. His growth rate has been on the slower end of the dogs I've owned. Because his growth was slower it caused him to put on a little too much weight at times and we had to adjust his diet many times. I kind of worried about pano with him as I had heard of older siblings (previous litter) and a litter mate having pano. He does have a higher pain tolerance but as far as I can tell he has never experienced pano.
So for me - I can't prove a genetic, growth rate, or dietary component. I guess all I can say is some dogs get it - some dogs don't.