You need to re-think that.
Even in DX11, there are some titles where CF/SLI has zero issues with latency. Higher frame rate and smoother.
Let's imagine a 60 FPS scenario, 16ms per frame.
1 GPU = 60 FPS = 16ms per frame.
2 GPU with perfect scaling (95% is possible) = 120 FPS = 8ms per frame.
The problem is when it's done poorly, GPU #1 and #2 are not in sync well, leading to big frame time variance.
All DX12/Vulkan mGPU does is give developers more control. If they are capable, the result should be better. If they are not, well, no mGPU support at all. :/
This is where you fail to understand how AFR works.
With AFR, every other frame you see was displayed by alternating GPU's. If you get 120 FPS, your frame times are 8.33ms, the same as with a single GPU getting 120 FPS, BUT there is one major difference. Each GPU is only creating 60 FPS, and each individual frame they create, takes 16.67ms.
Let's me see if I can create a visual for you.
[GPU 1 frame][GPU 1 frame][GPU 1 frame]
..........[GPU 2 frame][GPU 2 frame][GPU 2 frame]
While the displayed times are 8.33ms a part, the rendering process of every frame is 16.67ms.
With a single GPU, the displayed and rendering times are 8.33ms.