I would also suggest the 810A. So much more flexibility (uATX!) for just a tiny bit more space. I also went with the Seasonic 1U and used cable extensions - which makes things a bit messy, but short of custom cables, thats the way it is. I use an NH-L9i as well. My one complaint that U-NAS could actually address on the design would be to flip the location of the PSU. So many boards you want to use these with have the power connections on the CPU side rather than the back, they are basically forcing you to use extensions. If the PSU were on the right instead of the left, you might be able to skip the extensions. The other thing I'd love to see (although not needed I suppose) would be some sort of perforation for the cover above there the CPU sits - a little more air movement would be nice.
I replaced all of the factory fans with Noctua equivalents - the small side fan and the bigger HDD fans. With all of the fans going, I still hear more HDD chatter than fan noise.
For reference, my unit has 8x8TB drives (WD Reds and Seagates) and two internal SSDs, running off of an LSI card and mini-itx board. It is cram packed even with a mini-itx board. I've been running it fully loaded for about a year now, and HDD temps seem to hover between 25-35 degrees, depending on time of year (ambient temperature varies quite a bit). CPU temps are a bit trickier to track down using FreeNAS, but when I was starting out and watched it like a hawk, CPU temps were never a concern.