Didn't realize that, good to know. However, unless they added some magic sauce for it, gpu transcoding won't have as good of image quality (yet) as cpu transcoding image quality. Same reason I don't use quicksync on my i7 when I am handbraking my movies for plex.
Go to their forums and see what others are using. I've noticed that there seem to be a few configs they recommend and you may have a hard time troubleshooting an issue if you are not using their "recommended" hardware.
It doesn't take much CPU for NAS. You should be more concerned with bus and interface bandwidth than a CPU. An ages old CPU from the late 1990's and early 2000's will handle the workload, but wont have the buses for good throughput. Any of the Core or AMD64 processors with PCI-e buses will more than handle NAS duties.