I've found that the mia_physicalsky node settings that work for me are pretty much the defaults except the haze and saturation.
The shadow softness you can set on the mia_physicalsun node (Shadow softness). For my current model I use shadow softness of 2 with 16 samples.
On the mia_exposure_simple I set the gamma to 1 and for the render settings I set the gamma under quality->frame buffer to .555 (My monitor is calibrated for gamma 1.8) or .454 for 2.2.