Still, comparing NVidia's current lineup to AMD isn't apples to oranges. If you want a 1070 or 1080 just get one, Vega is still months away and while we see AMD stomping in Vulkan and DX12 benchmarks compared against similarly priced cards (RX480 vs GTX 1060) they don't have a proper answer for the higher end cards until Vega is out.
Otherwise, don't let driver support deter you, the whole design behind AMDGPU-PRO is much like NVidia's proprietary drivers - it lets AMD reuse the majority of their Windows drivers to keep parity between platforms; and, again, the open source drivers only keep getting better at a rapid pace.
amd is a bit lower, but you can use the built in open source driver and get roughly the same performance as the proprietary driver (From what i understand, the proprietary driver is going to only be there to support their direct clients, and in general, they are working on getting all generic improvements into the open driver).
From what i understand, this means kernel updates mean you have to re-build the nvidia drivers, whereas with AMD, the new drivers are included in the new kernel.
either way, I went with an AMD rx480 this month because i dislike nvidia's closed off business practices (especially in comparison to AMD's open practices). it runs everything I want it to run just fine, so its not like i suddenly cant play some game because i went AMD.
With the push to the linux kernel AMD GPU's would be better than Nvidia... that being said, I wouldn't hold my breath.
If you want to write a kernel driver that's effectively a Russian train toilet nobody will stop you, and you can feel free to maintain the engineering effort on it - but it's not going to be accepted into the mainline Kernel and you'll have to maintain it out of tree.
If AMD wants a shim to make it easier to port their Windows drivers over that's perfectly acceptable, but they need to work with the existing DRM infrastructure and the people that maintain it to get a kosher driver that everyone can be happy with.
That said, they were warned 6 months before the rejection that abstraction layers in the kernel would probably get rejected. The rejection wasn't news in any sense - Dave Airlie just decided to be more direct about it because the warning from half a year ago was ignored. No one knew that was what they were doing because AMD did all the development in private and then just handed over the code. Had they been more open and engaging with the kernel developers, they could have saved a lot of time.
There were a few emotional responses, and then it was made clear that they weren't done yet, and were still willing to work together. So everybody calmed down and got back to work. This all happened within a few days of the initial rejection, so you really shouldn't push the "it's never going in the kernel" narrative. Because much of it will probably be in there by the middle of next year.
It only interacts with performance in that driving displays requires memory bandwidth and consumes power.