Some of what you have said is accurate. However, Kia and all companies for that matter don’t say that their cars WILL get lower performance or fuel economy with lower Octane. Instead they say or at least imply that the cars may get lower performance or fuel economy in certain conditions.
The truth is that IATs affect knock tremendously. Those IATs can vary a lot as well. I have seen as high as 150 degrees while idling at the track but as low as 40 degrees while doing rolling pulls in near freezing temps.
If you want to absolutely guarantee that the car will perform at its maximum then by all means use the highest octane gas possible. However, if the Stinger is truly 100% stock and the IATs are very low then even 87 Octane gas will produce the same results as 93 Octane gas. I ran my best 100% stock 0-60 mph(4.59) with 87 Octane in cooler temps.
However, once I just simply replaced the stock
snorkel I could go much faster but only when I put 93 Octane in. I tried 87 Octane with the new
snorkel and I still could only get 4.59. With the mod and 93 I was able to get down to 4.37.
Temperature really really matters. As much as Octane matters. The manufacturers have to account for the car being used in extreme temperatures and that is why they have to recommend 91+ Octane. They have to cover their bases no matter what conditions you use it in.
Originally they only recommended 87+ for the Stinger but they changed it because it could pull timing in certain extremely warm conditions.