I wouldn't upgrade beyond 3tb. Some users have oveladed the sata power and developed i/o faults.
The technology is based on that of a DVD player. The sata was only meant for the power load of a DVD player.
Once you need more than 3tb your into NAS territory.
I upgraded my wan router to to the Asus AC88U some time ago. That has far more CPU power than the 1186 chip. I have a powered USB 3.0 drive connected to it. It copes to a PC are over 20 mb/s and hs no problem streaming to the Med. It also supports NFS 2.0.
Spending money on that was a bigger improvement to my overall system than spending the money just on a NAS.
https://www.asus.com/uk/Networking/RT-AC88U/I also have the 4 channel Asus wifi card in my main PC. That gives me a 1084 mbps connection via wifi to the router under my TV.