The amount of light emitted by the Sun is roughly constant (although it's affected by the amount of sunspots and the Sun activity in general). The amount of light received at the ground is far from constant -- it's greatly affected by the atmospheric conditions.
Making good reflections in metallic objects to emphasize the geometry might be hard from my experience. This is a very solid work from this point of view.
I'm amazed at how much faster the engine is - only took 20 hours to render to 5% convergence using an underclocked fx-8350 on bidir default settings, using a multiple scattering volume! You could've cut the rendering time by the factor of 2 or 3 (at least) by using single scattering -- without ...