Large terrain imported from Blender

Hi guys. I've been trying to look up some info on how much performance will be affected if I import a large terrain (with actual data exported from Sketchup) into Unity from Blender. It seems the Object2Terrain script only does a mesh at a time, so I use Blender to join the meshes before exporting an fbx file to Unity. The terrain will be about 10x10 km. Thanks for any input on this, of maybe a better solution.