Arma tends to be CPU limited - this is why it's normally suggested to avoid having any graphic settings turned down if possible, because most things when dropped to low settings end up being passed to the CPU rather than GPU. As both your CPU and GPU are shiny, and I presume your CPU isn't being stretched either, it's more likely that you're effectively running into the inherent issues with Arma: it's a relatively average engine trying to do a bloody enormous job.
As others have mentioned, the best bet is to set all your graphical settings to Ultra, and then drop your view distance to 1-1.5km as it is this view distance that chews up a lot of the potential frames. As a pilot with my flying view distance elevated to ~3km, I tend to have my frames fall off a cliff the instant I step into a helo