Speeding Up the game

Is there anyway to have grass and other details viewable from across the map, but have unity render them at a lower quality as you get farther from them?

You can adjust the distance at which trees change from their "real meshes" into flat billboard representations, but you can't do this for grass and the other detail meshes which are part of the details tab.

It is possible to add other items as "trees" however, even if they're not actually a tree model, and this would allow you to use the tree quality controls on them.

The built-in terrain system doesn't have that feature, no.