However I still think that Dx12 being "very close" to Mantle does not prove that it has been designed for GCN specifically

Again, I said it was "practically" (almost) designed for GCN due to its extensive roots in Mantle and the concepts within. I didn't say it was. It's not. But considering how heavily influenced DX12 is by Mantle, and considering that Mantle was written specifically for GCN, it follows that AMD was already well positioned for it. It doesn't follow that Nvidia, not designing their Maxwell architecture on one optional (async compute is not mandatory) feature of an API, that itself is an offspring of an API created specifically for an AMD architecture, means malice or incompetence on Nvidia's part. They were simply blindsided by AMD.

parallelization of compute task is an important step to better graphics performance. A fact Nvidia should have seen coming as AMD did years before

My previous post explains how Maxwell can do compute in parallel (which is good, because it's the basic principal of what a GPU even is - massively parallel compute beasts), but does it differently than GCN. Async compute isn't a magic bullet "free performance for everyone!" concept, and async compute isn't necessary to process compute in parallel. Except AMD's marketing team has done an amazing job convincing gamers that it is.

Async compute is about the ability to de-couple compute from the graphics queue to better utilize compute hardware. GCN frequently had whole CUs sitting idle in DX11 games; Mantle (and DX12/Vulkan) were designed to fix that problem. Maxwell already internally schedules compute tasks across multiple SMs and can 100% utilize its shaders. Maxwell can process compute and graphics, even if they're coupled, as fast or faster than GCN. The real gains in DX12 are the better CPU utilization and reduced overhead. It's a testament to AMD's marketing team just how much they've made people believe async compute is a mandatory feature of DX12.

QB's problems are developer, not GPU architecture, caused.