May be useable with some kind of LOD system.
yep, could be intruding make something like a 'material lod' that make/show a small area made of terrains to put on the surfaces really near to the camera with their texture, rendered on a second layer on the camera and blurred with the original scene:
placed in a 'room'/'portal' culling system that render only the small room viewed from camera, could be also usable (may with less polys XD: there's not too much difference between 800000 and 4000000 tris versions)