My advice would be to try with displacement... it can get you nice result and it is much more predictable, especially on such large surfaces...
Second thing, you need much more work to get perfect grass directly from render than to enhance it in postwork, so you may want to render simple displacement grass and than use some photos and textures to paint over it, do some collage untill you are satisfied with it... check this link, I posted it on forums some time ago, but here it is again, great images with lot of postwork...
Auralab • architecture+images
Auralab • architecture+images