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1) It's not what happens in real world. Let me explain: It's true that in real world, the shadows are not pitch black, but this is surely not because the sun change its properties. This is because there's light bouncing from other objects and from difused light in the atmosphere, which makes the shadows lighter. So, that's what we should aim at in Vue as well. There are many ways of doing this, here's some from the top of my mind, which can (should) be combined for a better effect:

• increase sky dome lighting gain (which simulates light that indeed comes from the sky dome in real world)
• make light balance at 30% (or any other value you like) (which simulates overcast skies)
• use glow in sky, fog, haze tab (which simulates diffuse light in real atmospheres)
• use radiosity gain (this is not a real world equivalent)
• use secondary directional lights (fairly dark) (could happen in real world, depending on circumstances)

2) Second reason for not using anything less than 100% in shadow density is that this is a source of noise in the shadows, in certain conditions. To compensate for this, you have to increase the quality boosts in several places, which increases render time.

Hope this helps! :-)

Very valid points, thanks a lot.
Actually, besides the overall better illumination, the car itself looked better in the first render because of the higher local contrast, but I didn't want to add even more fill-lights because I already use a ton of them.

I'll try your suggestions next weekend (too much work during the week).