I think Acetone is right. I have a 780 too, and I had no problems pushing the texture settings to ultra... But I don't see the point on doing it... I would highly recommend you leave the terrain settings alone (included terrain quality metric and max texture resolution). Outerra looks amazing with default settings.
For example "shadows low dither" looks really nice and, IMHO, some times even better (small objects shadows, like the buttons in the cockpit, are more defined) than "high dither" or "very high".
The only settings that I use to touch are:
Resolution.
Full screen (Most of the games works better for me in full screen).
V-sync (full refresh).
Anti-aliasing.... sometimes (FXAA is good enough at 1920x1080).
AO to high.... (mater of tastes).
And in world settings, if I'm going to roam at ground level, I push the grass distance to something like 90.