I don't know the extent, I just remember that Wags once mentioned that once you get away from LV the quality starts dropping. (those are not his exact words)
So the highest mesh is around Vegas and places like Creech, etc... Areas immediately around those places are equally to slightly less detailed and it goes out to the edges which are referred to as fly over country. Those areas have elevation data, and a simple mesh.
Again, that is not verbatim from ED/Wags or anyone else. That is my summation of what I remember the update covering.