500MB/s is 4,000mbps, so quite a lot, but not full power. Which model NAS are you using, does it have the base RAM or did you upgrade it, and how many of what disks in what storage configuration.
You may want to do Jumbo Frames on that interface - changing the MTU to 9000 from 1500. Is the NAS directly connected to the Mac Pro or thru a switch?