To somehow ease your pain centre part of Cyseal is the most CPU/GPU intensive place in probably whole game.. Can you please switch game in the centre of Cyseal to windowed mode (1680+ window) and launch activity viewer. Keep the game in foreground while looking at CPU tab of Activity viewer. How much CPU does the game consume?
Some generic notes:
1) Your game should be able to handle High textures (1024Mb of video). Performance should not drop
2) Turning off shadows is too much. Medium is fine. Try turning off Light Shadows for starters.
We will continue with GPU analysis when you'll be free. Do you have XCode installed?
