I think Kia is targeting the BMW 430i/440i Gran Coupe and the Audi A5 Sportback. They will then try to bleed sales of "other" versions of those cars and their competitors. Like the BMW 330i/340i, Audi A4, Infiniti Q50, etc.
Since Alfa Romeo is targeting the 3 series, A4, C-class, etc. with the Giulia I don't see why the Stinger couldn't also be in the mix. The base 280hp 2.0T Alfa Romeo Giulia could be cross shopped against the base 2.0T Stinger. The forthcoming Alfa Romeo Giulia Veloce, with the 350hp version of the 2.0T, seems to align with the Stinger GT as well.
All the factors listed previously (performance, price, interior room, trunk space, price*) will come into play if someone keeps an open mind when looking at both cars.
The Quadrifoglio is a whole different animal and therefore out of the discussion here.
* Yes, I mentioned price twice