How about the
Synology DS1812+ instead to future-proof the setup with three more bays? Sounds like your NAS would be pretty full right from the start, and with the 8-bay, you can add the other three drives later on when the need arises.
The Synology NAS devices have what's called "Synology Hybrid RAID", which is similar to Drobo's BeyondRAID.
Also keep in mind that the Plus-models of both the 5- and 8-bay devices do link aggregation to double (and even quadruple on the DS181
3+) the transfer speeds via the two Ethernet ports on your Mac Pro and two on the NAS, but you need a router/switch that supports the 802.3ad standard. The
Cisco SG200-08 and
Netgear ProSafe GS108T would be two of those switches.