Game only using 20% of all GPUs?
I have been running benchmarks and all four of my 580s never go above approximatley 20% GPU usage? I am running the latest drivers.
ArmA 2 is CPU limited, even a single GTX580 hardly reaches 100%, more likely around 80-90% which fits with your GPU load.
Besides this.....what is the rest of your system, what settings do you have ingame...you know, the usual uninteresting stuff.
single GTX560/GTX570 is maximum needed for Arma2 GPU, IMO.
probably in next versions, GPU's was put into good use -)