Some things you can lower to get improvements in FPS is Reflections, they can really bring down frames when it rains as when things are wet, they reflect more, there is almost no difference between high and medium shadow details, only high takes more from your system, particles is something you can lower as well, you barely see them only when you or other cars crash.
Bloom, turn it off, it eats away at your frames, same goes for Motion Blur. Grass can be lowered to medium because when do you really see the grass? Most of the time you are going 100 km/ph.
Also, the higher the AA the more it will eat at your CPU and make it work harder (the reason why we see allot of people with AMD CPU's complaining about higher temperatures), the physics engine in the game has been proven to be put more strain on AMD GPU's as Nvidia has implemented their Physics engine in the game so more cores are used while with AMD it uses one main core and stresses it out. Hope this helps out...