I think that ThinWall ( Diffuse ) Transmission type works wrong. If this was made for vegetation, leaves type of materials it should "transmiss" light only from direct sunlight - not from ambient ( or very little subtle from ambient ). Please go outside on evening and see how tree leaves looks
This is how it works without sunlight :
And this is how it should looks ( standard thin wall transmission ) :
This should works more like ThinWall but without transparency effect - transmit light only when camera is oriented directly to polygons that have sky behind.
I noticed that this works fine without sunlight using very small Transmission values. But after that if we want the same material to looks good ( with very good visible grass, leaves transmission effect ) using strong sunlight there is need to crank up Sun value to 1.5 or 2.