Render time was about 2.5 hours @ 1000x1300px on a quad core Mac Pro
Yes, Carrara has an instancing system called the Surface Replicator. You can define zones on the terrain with shaders that determine where the instances go.
Now, about the water that everyone seems to have a problem with. I'm reasonably happy that I hit the "look" I was aiming for. Here's a link to a photo with some water flowing that is a similar size, depth and speed to my scene.
You can see that the water is quite rippled but doesn’t quite cross over to splashing and foamy. However, the large number of comments on my water obviously means that it’s something I need to work on some more.