LOD makes a HUGE difference already, and I have even started lowering the polycount.
If your terrain is one object, all polygons will be drawn all the time. If your terrain is multiple objects, only those objects in the view frustrum will be rendered. So already we only have to render parts of the terrain that are directly in our view, and within the camerea range. That can cut the poly count down to 25% what it was before.
On top of that, and I haven't done this yet, each terrain piece gets a low-poly version that is used when the camera is far away. So you can crank up the poly count for the terrain chunks near the camera, and use really low-poly terrain for pieces in the distance.
If your terrain is one object, all polygons will be drawn all the time. If your terrain is multiple objects, only those objects in the view frustrum will be rendered. So already we only have to render parts of the terrain that are directly in our view, and within the camerea range. That can cut the poly count down to 25% what it was before.
On top of that, and I haven't done this yet, each terrain piece gets a low-poly version that is used when the camera is far away. So you can crank up the poly count for the terrain chunks near the camera, and use really low-poly terrain for pieces in the distance.