He is talking specifically about the 2009 Mac Pro flashed to 2010 firmware, not the stock 2010/2012 models. If there is a specific post that mentions someone testing that scenario then please link it directly rather than point to a 7 page thread.
Apple says No
I say Yes
View attachment 764550
(I had a quick look at the GPU drivers everything pre-metal has been purged, interesting NVIDIA fermi drivers are still there so does Fermi actually support metal? i Remember that being a bit debated "back in the day" no NVIDIA drivers newer then kepler tho sadly)