You go from 16 terabytes to 14.5 tebibytes right out of the gate. And then if you use Raid-Z2 (Raid 6), which uses 2 disks for parity, on a four disk set, half of your drives will be used for parity. This was of course the right call, because with the size of current drives Raid-Z1 (Raid 5) is really risky.

Personally on my ZFS based home built server I use mirroring, due to the well publicized issue with the extreme difficulty of increasing the size of ZFS's VDEVs. Which has the same 50% usable space reduction.