AC4 is a very demanding game, so I'd say it's probably normal.
Hard to compare to the "other guy" since we'd have to know if he had the EXACT same settings and level in game etc.
Here's a benchmark you can look at which seems to suggest your results are normal though it doesn't list the specifics enough to repeat:
http://www.hardwareluxx.com/index.php/reviews/hardware/vgacards/29094-test-asus-geforce-gtx-780-ti-directcu-ii.html?start=17
*Here's how I tweak all AC games:
1. VSYNC OFF
2. Tweak settings until above 60FPS most of the time
3. Force Adaptive VSync on in NVidia Control Panel per game (Manage 3D Settings-> ....)
Now the game should run at 60FPS VSYNC ON if I can output 60FPS+, otherwise VSYNC is disabled automatically if I drop below 60FPS.
If the SCREEN TEARING becomes too frequent (dropping below 60FPS) I go back and play with the settings (lower Shadows or whatever) to raise the frame rate a bit.