The big issue here is that MacOS is a proprietary and closed fork of Darwin which is a BSD. MacOS has many improvements that Dawin has not...
This is a part of the root of the BSD/GNU GPL flamewar.
"Thanks" to the BSD license, apple can improve and close the code witout publishing it. MacOS is closed and proprietary. With the GNU GPL, fairness (open source) is protected, enforced: you cannot close the code. It allows us to go after optimal code.
Most of GNU GPL coders are looking to have an *optimal* open source OS installed on systems. The GNU GPL helps towards that goal, but is not a silver bullet.
Regarding the GPUs drivers, since they are BSD, apple can rip that code, share secrets with nvidia/AMD/intel... in order to keep the optimal code for MacOS crippling the open source version (they did it for NTFS support for instance).
I do remenber BSD advocates saying that no company will be able to have a better proprietary fork of a BSD OS. They were wrong.
So what you want is to rip out the close source drivers from MacOSX and install the open source drivers?
Doesn't the 3D work very well out of the box with Mac?