As i'm a fan of openwrt router (WT3020H = 15$), i will give a try to link my pi with usb-serial pl2303 (hosted on router) at highspeed (1mbit fullduplex for sure) over tun application.
In theory pl2303 support 3mbit serial link, some part from prolific claim 12mbit.
In this case, the openwrt router give 2 ethernet and wifi mimo.
I think it's a better situation than arduino yun, for hard computing, less expensive (25$)...