Prime , I may be missing something but the Grass in your last shared rendering looks rather nice - can you let me know exactly once more what is bothering you? Is it the difference in brightness of the Grass over the larger areas? If so, then this may simply be clouds in the sky casting shadows - you can dial down the cloud density whenever you like in the sky tab of our Visual Settings. If that is not what is bothering you, can you briefly describe it with the last renderings as the main example?
Ok .. I'll try to show you what I mean exactly when I get home.