Try turning OFF Occlusion culling. With large entities (planets, large ships) turning it off results in a rather big speedup.
Looking at a full loaded r600 planet, i get:
12-15 fps with Occlusion Culling ON
80+ FPS with it OFF
(GTX 970, 4gb vram, 8gb ram [6gb to SM],
[email protected] ghz)
Good luck!