Yes the procedure is the same for nsa310 and 325, but it's better if you do it as I say now as the original procedure does not deal with cases where there are bad blocks in flash (and I had another user with bad blocks that pointed this out).
All kirkwoods are using UBI with kernel+squash+Ubifs in it.
The squashfs root partition allows "reset to default" functionality and it was deemed more useful than saving a few MB on devices that have like 128 MB or more available.
There is an image for 325 too, it is currently only in development snapshot, so be aware that you won't be able to update some packages soon after you installed the device, as is the case for all snapshot builds https://downloads.lede-project.org/snapshots/targets/kirkwood/generic/
As usual, it might be less frustrating to make an image with Image Builder with all packages you need and then sysupgrade with that every now and then.
Follow the tutorial here http://forum.doozan.com/read.php?3,12381 to backup all stock partitions and install uboot (it also provides tools). The image that you must use is the "u-boot.kwb" one. Ignore the part of the tutorial about flashing envs manually, the LEDE uboot has them compiled in already.
Then reboot, press a key to stop auto-booting and add new env for the mac address so it will be initialized with the right address.
The MAC address is usually on a sticker under the device (one of the two codes is the serial),
it should begin with "107BEF" as it is assigned to ZyXEL.
write in the u-boot console (use your MAC address instead of the example)
setenv ethaddr 10:7B:EF:00:00:00
saveenv
Download with your PC and place http://downloads.lede-project.org/snapshots/targets/kirkwood/generic/lede-kirkwood-nsa325-initramfs-uImage in a normal flash drive formatted with FAT32.
Insert the flash drive in a USB 2.0 port of the NAS (back ports), then write (copy-paste) in the serial console:
usb reset; fatload usb 0 0x800000 lede-kirkwood-nsa325-initramfs-uImage; bootm 0x800000
This loads that firmware image and starts up a LEDE firmware that runs from RAM. When it's done starting up, do a sysupgrade as normal.
cd /tmp
wget http://downloads.lede-project.org/snapshots/targets/kirkwood/generic/lede-kirkwood-nsa325-squashfs-sysupgrade.tar
sysupgrade lede-kirkwood-nsa325-squashfs-sysupgrade.tar
This takes care of reformatting the internal flash with ubi and dealing properly with bad blocks.
After the reboot it should be running LEDE from flash.
Currently the Sata and USB leds work but are not used, but there is a PR where I enable them (on both the 310 and 325) https://github.com/lede-project/source/pull/938
It will eventually be merged, I hope.
Btw, this procedure is the same for all kirkwood devices, just adjust the file names.
I'm not in my best shape.