The Raspberry Pi doesn't have any high-bandwidth peripheral connections, and the CPU is too weak to do high-speed routing with QoS. What's needed is an open source board with a modern CPU, a mini-PCIe slot or two for the wifi, and at least one or two GigE NICs. In other words, this:
http://www.pcengines.ch/apu.htm
Although once you add the wifi, case, power supply, and SD card or mSATA SSD, it's a bit on the expensive side.