It does appear that we have proof that they are different parts, and likely not off the same assembly line. Now the question is whether or not it's worth it. As I pointed out above, the $50 part is mid-tier according to GM.
On to the philosophical discussion...
I'm not sure how GM does it, but in the defense industry, demanding 5-nines reliability is one of many reasons stuff aint cheap.
The consequences of a part failure in military aircraft is far higher than for our trucks. Still, you have to have a critical eye when a vendor says they can improve the quality.
Gray iron is 90% of cast iron production because it's strong enough. I'm less worried about the material and more worried about the casting process. Substituting a higher grade of iron when the cheaper version works is not quality, it's expense. Our water pumps have bearing and seal failures, but I haven't seen a casting failure yet, and if I did experience one I would blame a casting flaw instead of a material inadequacy.
When I parted out my '87 Jag XJ6 to build it into a race car, I was amazed at both the expense and lack of quality in its construction. The interior door handles weighed a full pound apiece. I followed one wire circuit from the front of the car, to the back, to the front, and to the back again. It was a 4000 lb car that should have weighed 3500 lbs. They did these things out of a rationalization that they were improving quality, but it you remember, Jaguars were notoriously unreliable at the time. Spend the money where it makes a difference. Jaguar only improved their quality by getting bought out by Ford.