I am getting low FPS as well it does improve a tiny bit when I do auto detect but I can run many more powerful games than Arma 3 on ultra such as Battlefield 3.

PC Specifications:

8 Gigabytes DDR3 RAM
AMD Phenom II x4 955
Nvidia GeForce GTX 560

The FPS stays around 20 or 25 for me depending on the area.