One way is to turn on soft shadows. You can then set it to Premium at 4 RPP for your test renders. It's much faster that way than on regular AA when you soft shadows enabled. Also, you might try rendering the texture from the bird's eye orthographic view and using that as a height map, as in the demo pic. For the final product, I was too lazy to use soft shadows and wait for it to render (doesn't look too great at 4RPP lol). Hope this helps.