IMO ARM is kind of a wash when it comes to NAS - with modern chipsets most of your power goes to keeping the disks spinning. My C2550D4I file server build I just completed uses about 60W in idle. By my calculations ~40 or so of that is the power used by the 8 disks plus SSD boot drive.

x86 chips are more then suitable for the application since you're no longer in "ultra ultra low power" territory (and for ZFS, are beneficial because you want those checksum calcs to finish fast).