Off-shelf NAS device will be not only slow but also full of various bogus bugs with which you need to wait for vendor to issue firmware update...
Just build it yourself - build a PC. You have plenty of options:
1. If you have a rack somewher buy a low end rack 2U rack server with enclosures for SATA disks and some decent RAID controller.
2. Build yourself a PC in tower enclosure. Get some Core 2 Duo mobo (cheapest), medicore ammount of RAM - SMB and NFS and AppleTalk servers with Linux operating system will eat up something like 80MB for the system and 10MB per client computer - go figure, the rest of RAM is for I/O buffers. Stuff as much as you can get SATA disks into that (like 4x 1TB). Setup it with software RAID. And you are done with it. Probably it will be much cheaper than decent NAS box (so called SoHo boxes are no worth even looking at).
Do so and you have a decent storage that is more efficent that your network.
You said about network efficency? Well - this has nothing to do with NAS box. You can have the best performing NAS box - but if your network is weak - well here goes your efficency.
So as for network buy managable switch that can cope with Linux channel bonding - with that you can bond N ethernet channels and get network transfers somewhat lower than N*interface speed.