Updated info on the lower-than-expected performance Mohns saw in his testing. The bottleneck is NAT:
In our testing, AirPort Extreme provides outstanding 802.11n performance through its LAN ports. However, when using Internet Sharing (also known as NAT, or Network Address Translation, mode), throughput via the WAN uplink port drops considerably. We observed a maximum speed of 34 Mbit/sec. in this configuration. While this is faster than a typical cable modem or DSL line, it’s considerably slower than a switched 100 Mbit Fast Ethernet connection.