Neither Vulkan nor DX12 are just Async Compute over the previous one. They are whole new APIs which explizit control over the GPU (memory management, draw calls etc.). Without optimizing they are not better.
Right Oxide has designed an engine which is not optimized for nVidia and they are just doing nothing but disabling certain code paths. Isnt this what i am writing?
DX12 allows support for Multi-threaded rendering, Multi-engine, multi-adapter etc
NVIDIA already made good use of multi-threaded command listing and deferred rendering under DX11. NVIDIA used hidden driver threads to boost CPU performance and thus lower their API overhead. This resulted in a higher draw call rate. This is achievable due to a few design perks, one is that since Kepler NVIDIA have been making use of Static scheduling. Static Scheduling means that a large part of your scheduling is done in software. NVIDIA multi-threaded their driver. So several Threads were feeding NVIDIAs hardware. Secondly the gigathread engine allows for larger batches of commands to be fed to NVIDIA GPUs. In essence, NVIDIA GPUs have a much larger command buffer. This is why you're under the illusion that Maxwell is superior to GCN, you never factored in the additional API overhead GCN entails under DX11 by its use of hardware scheduling.
Moving onto Vulcan and DX12, this API overhead has been lifted, giving us a glimpse of the true performance of GCN, Kepler and Maxwell.
On top of this GCN is a much more highly threaded architecture (more parallel). So GCN has a lot of untapped resources on hand. By using Asynchronous compute, you can maximize your applications usage of GCN.
When you couple both together, GCN overpowers Kepler and Maxwell. Why? Because GCN is a better architecture.
Maxwell may have the upperhand it terms of its front end, compute wise however... Maxwel isn't even close.