It's more efficient to use a small hardware router for NAT. They make forwarding decisions at the hardware level, so therefore they can make them with less latency, and consume less power.

You aren't actually creating a machine with 7 ethernet ports. There is still only the throughput of one. Also, if you are doing major file sharing, you will run into collision hell if you're using a hub.

In sum:
Don't use software NAT unless you really have to.
If you have a lot of traffic from multiple nodes, don't use a hub unless you really have to.