I'm considering this too (since indeed one significant part of heat comes from the disks---due to the size of data served and backup'ed, spinning disks are the only reasonable solution. But I'd like to give ARM a try in order to not have all eggs in the same basket and to see how this works.
It's always good to have some diversity :)While not ideal for high speed or multiple disks, I've set up a few Raspberry Pi file servers and wrote about some shortcomings and how to overcome them here:
https://www.reddit.com/r/raspberry_pi/comments/a6ndx0/review_of_suptronics_x830/ John