DX11 yields better performance for me than DX9 in Far Cry 3. Other have reported this as well... for some DX9 is better, its a toss of the coin, the game's performance is all over the place.
That said, 1680x1050 w/ Ultra settings, HBAO, advanced A2C, MSAA 4x - i get from 50 to 85fps in...