For FCPX that is a good card. You can add a 2nd and FCPX will actually use both GPUs for rendering your videos. I went with the R9 as I don't know that I want to have 2 GPUs installed, taking up PCI slots for other upgrades I want to install.The radeon hd 5770 that came stock with the 2012 mac pro.
Overall the system is quite quick, taking the throne away from my 2015 rMBP (AMD Graphics) with regards to FCPx speed. The only thing that is really bothering me now is the slow disk speed of the native controlller. I know it will never reach the speeds of my MBP, however, to go midway would be awesome.