DirectX 12 support is coming in the Tanoa expansion next year, so there's that.
GPU improvements have nothing to do with how Arma 3's game engine "processes" in game performance.
The game engine was designed to utilize cpu power rather than gpu power and efficiency.
I've said as much in many other threads. However,
DX12 does allow the GPU to talk to all CPU cores simultaneously, so it does allow for a form of CPU multithreading. There should be some sort benefit, however limited, to implementing DX12. Given the amount of work involved in adding DX12 support, I don't believe Bohemia would bother if there wasn't some sort of benefit.
But DX12 isn't a magic potion, either, so even if it does yield benefits, we're still going to have performance issues.