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.
With the push to the linux kernel AMD GPU's would be better than Nvidia... that being said, I wouldn't hold my breath.