Try to haggle with your dealer a bit. Tell him another person got S7 installed, also a Kia Stinger, see his reaction.
I have not really researched on the tint much. I believe I got S7 installed on the front windshield and front windows and S5 on the rest. If you can wait until the evening, I can take a photo of the receipt. I remember them saying it comes with a lifetime warranty. You can use it to help you haggle with your dealer.
I really don't know the difference between each series. We didn't talk about which series to be installed, I just asked if they have 3M, but the shop only carries Rayno. I didn't want a dark tint, so I went with the lightest they have (front windshield and front windows, a step darker for the rest). I think the shop didn't have lighter tint in stock or not enough in the darkness level I wanted, so they automatically 'upgraded' to S7 for the windshield and front windows. I just have to wait until summer and really see if the film is any good in the strong sun. Clarity-wise, I don't see any difference.
Not sure if this helps much but that was how it went for me. My wife was mainly focusing on the dash cams, she really wanted to get the best model the dealer can give and we didn't spend much time/effort on the tint.