Arma 3 is incredibly CPU-heavy, like Arma2/DayZ. At least, it has better multi-core utilization over the older titles. Since Kaby Lake apparently has essentially no improvements to IPC from Skylake, and you already have a 6700k, you may as well overclock it balls-to-the-wall and see if your fps doesn't get any better. Beyond overclocking, getting an SSD, getting more RAM, and getting a GTX1080, you don't really have any worthwhile upgrades. However, getting a more powerful GPU probably won't net you any FPS gain, especially if you are already completely CPU-bound (which is most likely).
You should run some monitoring software like Afterburner, and see which is being used more, CPU or GPU. If your CPU is being used way more, CPU...