Far Cry 3 CPU Usage

nope thats normal. fc3 likes quad cores and 95% usage is perfectly fine (you dont want it to go to 100 or less than 80 because then the cores arnt being used effectively)and you should be getting 30-70fps with them cards in xfire. if it gets a little jittery play with the render ahead settings in the video options set it to 1 or 2 and the game should smooth out. if its getting a lot of micro stutter then vsync is your only real option. but you will have to get up to 60 fps for that to work properly.(drop the settings)
the 1 thing you can do to dramatically increase perfomance and in my opinion increase image quality is turn off post processing completley)turn it to minimum) this gets ridd of the radial blur which i dont like...