I know the 780 ti is better - I'm not turning vsync on and I do have a i 5 3570k...
Yes you have a bottleneck however even with a 4770k you will not get a good frame rates with black flag maxed out with pysX @ 720p...
Part of the problem is the the game is new and it has lots of tech that needs to be refined below is Nvidia's recommendations that were posted before the physics patch came out for 1080. " when enabling pysX in this game your get huge frame drops...
The game still looks good with out pysX and maxing anti aliasing - You will probably have to disable God rays because they require lots of compute power...
The game is so full of tech that Nvidia recommends their 1000 + Titan use fxaa at 1080p...
Check out the link, @ 1080p Nvidia
http://www.geforce.com//whats-new/guides/assassins-creed-iv-black-flag-graphics-and-performance-guide
1920x1080 AA AO Env. Quality God Rays Reflections Shadows Textures Vol. Fog
GTX TITAN FXAA HBAO+ (High) Very High High High Soft (Med) High On
GTX 780 Ti MSAA 2x HBAO+ (High) Very High High High Soft (High) High On
GTX 780 FXAA HBAO+ (High) Very High Low High Soft (Med) High On
GTX 770 FXAA HBAO+ (High) Very High Low Normal Soft (Med) High On
GTX 760 FXAA HBAO+ (High) Very High Low Normal Very High High On
GTX 690 TXAA 4x HBAO+ (High) Very High High High Soft (High) High On
GTX 680 FXAA HBAO+ (High) Very High Low Normal Soft (Med) High On
GTX 670 FXAA HBAO+ (High) Very High Low Normal Very High High On
GTX 660 Ti FXAA HBAO+ (High) High Low Normal Very High High Off
GTX 660 FXAA SSAO High Off Normal High High Off
GTX 650 Ti Boost FXAA SSAO High Off Normal High High Off
GTX 650 Ti FXAA SSAO Normal Off Normal High High Off
GTX 650 FXAA SSAO Low Off Normal Normal High Off
GTX 590 FXAA HBAO+ (High) Very High Low Normal Soft (Med) High On
GTX 580 FXAA SSAO High Off Normal High High Off
GTX 570 FXAA SSAO High Off Normal High High Off
GTX 560 FXAA SSAO Normal Off Normal High High Off