DX12 - GCN 1.0 Cards have negative impact on performance

Hi there Dice.
I am ObscureAngelPT, I own an amateur Youtube channel related to benchmarking.
I do benchmark my 2 home PC's (a desktop and a laptop)

Both low end stuff.
In my desktop
I have a Phenom II X4 945 at 3.6ghz 8gb of DDRIII 1333mhz and a HD 7850 2gb.

The game runs absolutelly fine, but it's entirelly bottlenecked by the CPU.
So CPU Usage is always at 100%, still I get acceptable frames between the 40-70FPS on medium settings.
All this under Dx11.

So, I was expecting that DX12 could help me out with the framerate.
I did used Mantle in BF4, which I had the exact same problem as on battlefield 1 with dx11, and it did improved a lot the performance which makes sense since, low level APIs, removes a ton of overhead, so CPU will have more room for work in other stuff..
So on DOOM vulkan helped me as well, Rise of The Tomb Raider with dx12 improved massivelly on the open world areas, where my CPU was bottlenecking with DX11

So I was expecting the DX12 to had similar effect on performance just like any other title, to drop a little the CPU Usage, ending my bottleneck and enjoying improved performance
But in the end activating DX12, still gives me a CPU Usage of 100%, a little more load on the card, and the same or worse framerate, and adds stuttering.

I have seen a video of digital foundry showing the improved performance that the RX 480 benefits from the DX12, maybe mostly because DX11 was bottlenecking the RX 480 due to the fact that AMD is just bad with DX11.
But in the end the RX 480 really benefits from it, which is a GCN 1.3/4 card.
My question is if DICE will work a little more and improve the DX12 overtime for older cards like GCN 1.0?
It doesn't really make any sense, DX12 should had helped me, a lot, because I have a very big bottleneck because of the CPU, this is a situation where the DX12 should had improved a LOT!
And it's making it so much worse.

Cheers