I think what I'm contemplating is simpler than that. (but I could be wrong
https://wikidevi.com/files/Atheros/spec ... R8327N.pdf
is the sort of device I'm thinking of.
I think that is capable of handling all the switch level functionality, even nat translation in the case of the N variant.
The data flow from the majority of users WAN connection is well inside 100mbit, so if it is possible to implement a 100mbit capable port from the GPIO bank on the Pi to interface between the Pi and the Atheros, with presumably an additional path for talking to the control registers on the chip for set up, the Pi could provide routing and presumably firewalling for the upstream traffic, without becoming a crippling bottleneck on the local LAN traffic.
As I mentioned, using a USB gigabit LAN interface on a Pi to attach a Gigabit switch works, for some definition of works, but makes the Pi even more of a bottleneck, because the USB path is used both in and out.
It is surprisingly functional however, I'm using a pi with a serial usb connection and a gigabit Lan interface, as a tool for remote set up of Cisco switches, very successfully.
Pi runs Peter Anvins tftp server, I can set up a minimum config on a "raw" switch via its serial console. (ser2net works ace), bring up a private Ethernet from the switch to the gigabit adapter on eth1 on the Pi. and run software upgrades and install configs. All done remotely via the Pi's eth0 interface, with the Pi keeping all the set up traffic off the real network.