Well, if you want to keep it simple, most would recommend the following package, which contains everything you need to make roughly 450 whp:
https://burgertuning.com/collection...orsepower-kia-stinger-hyundai-genesis-package
If you want more, to that package you could add a water-meth injection kit, also available on the same site. Most of the heavy hitters are running WMI, which has helped get them deep down in the 11s in the quarter mile.
https://burgertuning.com/collections/kia-hyundai/products/kia-stinger-genesis-water-injection-kit
If you want more sound, you can purchase a catback exhaust and/or midpipes. The debate is out on whether they make any useful power but personally I saw a noticeable improvement after I combined both. Another option would be primary downpipes and high flow cats. You would want to combine them with a tuner though, to avoid overboosting. You can search through various options on either
k8stingerstore.com, or
khartunerz.com.
The other big go-faster option is an ECU tune, but that would require removing and shipping your ECU, unless you live close to one of the tuners.
If you're still on stock brakes you will likely need to upgrade at some point, since the Stinger is known for brake pad deposits on the rotors causing vibration. Upgraded pads and rotors are a good idea, and can be found on either of the sites mentioned above, and others like
R1concepts.com.
Hope that helps a little but we're here to help if you have further questions!