Unless I look at the sky, or ground both GPU and CPU usage doesn't go above 75%.

Look straight at ground level, GPU and CPU usage can be as low as 25%.

I would expect that the game would almost always utilize at least GPU at close to 100%, and have higher FPS.

When I look at sky or ground, GPU utilization goes to 100%.

Specs:

2x GTX 560 Ti OC SLI (Arma3 profile)
i7 2600K 3.8 GHz quad core
800 MHz CAS 8 RAM (20 GB)
High settings, 1920x1200 resolution