sfrv, I think you are wrong in assuming we all have 2009 MacPros. I for one have a 2008 3.1, and I suspect others have older (and newer) machines.
I still suspect it is a lack of support for Nvidia, rather than ATI, cards at the root of the problem, largely driven by "political" forces at the top of Apple and Nvidia at the moment. I think the absence of Nvidia options in Apple offerings across the board since 2015, with the discontinuation of the 27-inch non-retina iMacs, is telling. And Apple has developed its own video acceleration hardware for its portable computers, or uses the built-in Intel chipsets, so BOTH ATI and Nvidia no longer have THAT part of Apple's business.
I note the proprietary video card slot on the new MacPros will make it difficult for 3rd party manufacturers to make lower priced offerings for the machine, or offerings at all, and all other Apple machines now don't have separate graphics cards anyway in them.
I've turned off separate spaces on my MacPro, and my El Capitan system is rock stable now, which has a dual ATI Radeon HD 8770 running two Apple Cinema displays, with a third being run by an older PC Nvidia GeForce GT 240.
I was intending to replace the Radeon card and perhaps even the old Geforce with two new Nvidia cards (Geforce GTX 750Ti s), but watching this thread develop, I have held off on that purchase, and are thinking of finding a low power new Radeon with similar performance to the new GTX 750Ti's, although it means the end of dreams of fast CUDA performance on the machine, but it might mean the return of separate spaces in my workflow..
I work mainly in 3d graphics on my computer, and there is one advantage I have noticed in El Capitan, which is the support of the new 3d API "Metal" at the Finder level, so my 3d objects in .obj, and other standard formats now have picture previews, and open up in preview/quickview as rotatable 3d objects, even large datasets, which is very impressive, and it all seems very fast on my machine.
I'm wondering whether my next machine won't be a Mac at all, but a Hackintosh running the dual version of the chipset in the current MacPros, although I have seen nobody doing it yet publicly, maybe Apple have expressly not supported this option, and we will have to wait for some creative Russian programmer to get around it.
As a "3d person", for a long time there I have been PC based of necessity, but MentalRay's significantly faster performance under non windows OS's had me change a few years ago to Mac, and I love it - my older Hp workstation (xw8600) which was my bread and butter machine under Windows 7 64 for a long time, now runs a rock stable cut of Mountain Lion, and is still very productive. http://appleinsider.com/articles/15/01/23/how-amd-and-nvidia-lost-the-mobile-gpu -chip-business-to-apple-with-help-from-s…