I looked into this A LOT before buying. I was all set on a Q50S (not RS) AWD and had a great deal on one.. Except that the sport seats dig into my legs in a weird way, and I just can't sit in them. They're better than the G37S sport seats - those are made for Lilliputian Jockeys. Plus, Q50 infotainment sucks, to put it mildly.
Stinger has Tork (google John Bushbaum's history / reputation before deciding if it's something you can live with), Lap3 JUST announced they can do full tuning and that's mostly it for tunes. There is Pressertech, but they don't do active development and it's pretty much done-and-over-with as far as their tune - stage 1 is all there will ever be unless things REALLY change. No one else has followed through on a decent tune, AFAIK. I guess there are the Motec guys, if you really want a fully customized Stinger. But that's going to take a LOT of DIY work.
Then, you have jb4 from the piggyback side. ~450 whp, maybe a bit more and if you do meth, maybe a bit more than that. But, launching is either super slow (OEM) or a gamble (brake snip).
With the Q50, true flex fuel is a thing: they have pumps, injectors, fully tuned ECU (ecutek), etc. AMS tuning sells various stages, you can get a fully built transmission, upgraded turbos, etc. So, for really pushing things, Q50 is better and probably cheaper. AMS built a 10 second Q50 for testing - 800 hp (whp?) with upgraded driveshaft, etc. They can definitely get you into the 10s.
Of course, after dropping $15K in
mods on a Q50, you may wonder if it was the right choice..