This is gonna be more windows centric as I game on windows, but I do use dxvk so it's probably not going to be that far off.
I5-1135G7, iris xe 80 eu, 16 gb of ram. I got around 40 fps in the benchmark on dx9, 50 fps on dxvk. This was on older Intel arc drivers. Vulkan performance has been consistent on their drivers but there may be some improvement. I was running 1080p medium settings, no anti aliasing and tri-linear as filtering.
I had an old i7 8550u laptop with a Radeon 530 2GB DDR3 graphics card. It ran GTA IV about the same with an older version of dxvk, but it had some performance issues because I couldn't find a higher wattage adapter for it and would throttle.
ALSO TURN OFF SHADOWS. For whatever reason, it's handled by the CPU.
Well, DirectX only managed to mostly catch up with Vulkan with DirectX12, so translating DirectX11 to Vulkan will likely improve the performance because Vulkan is a considerably more optimized API. GTA IV uses DirectX9, so the performance gain with DXVK can be quite big.