I think you can go further into the rounder looks, compared with those refs you posted on PJ. I'm sure this must be incredibly difficult to get right in iso.
The right fender (cars') disappears completely in the 'almost-iso' photograph. The bumper is more curved. The back is much more plunging (in the side view), which is a distinctive feature of Saabs.
The windshield corners are more rounded too.
Input on how to improve curvature using shading is still welcomed.
To be honest I didn't have very specific ideas about that!
Looking harder, I can see at least one point: those lines (shades and reflections) you have on the side are very straight. On the 'iso' photo they help a lot to define the softer volumes. Same thing tor that dark shade on the left (cars') side of the roof, it's too close to the side and gives a feeling of a flatter roof than it is.