Well here's my unsolicited $0.02... we all have opinions, right?
I would never consider myself a "fanboy" but I *have* been VERY happy with the Kias I have owned for the past 8 years. It was the 2011
Sorento made here in GA that originally sold me on quality and value of the brand.
I have driven an R8 - on a track. I also drove a Lambo, and would take the R8 over that in a heartbeat.
Obviously the (V10) R8 is higher performance in every regard, but I'll tell you - you can't use anywhere near those capabilities on the street. You can barely use what the Stinger has to offer. The interior quality is about equal.
I originally looked at the BMW M2, but was too small so considered M3 or M4. I also looked at comparable Mercedes AMG cars, and the Lexus RC-F. I was willing to spend up to ~$100k. I drove them all and was leaning towards the AMG because although the M cars had "better" performance, they had more than could be sensibly (and legally) used without drawing attention. Plus, they look average compared to the Lexus and Mercs.
Then I heard about the Stinger and, as I didn't *need* to buy anything at the moment, decided I'd wait and see. Drove it and love it!
Interior may not be as good but if you sat someone down and told them it was an Audi or Merc or BMW, very few would argue. There's hard plastic in some places, less than perfect fit in others, but overall the interior is VERY impressive. Caveat - the newer, higher-end Optimas, and most of the Genesis models, are a grade above and probably as good as the German brands.
Performance is comparable, BUT upgrading is cheap and appears to be getting very popular, hence many options.
So even though I can go out and buy any of these today, I'm happy to wait until I can get the Stinger with the options and colors I want. In the meantime, my GF just bought a beautiful (and sporty) CLS400, and we're both eager to spend time comparing it to the Stinger, when I get one..