Using the current images, I did a few measurements.
I run a direct connection between an Intel GBit Ethernet and the Parallella, and ran the following commands:
on the laptop: "dd if=/dev/zero bs=1M | pv | nc parallella 12345"
on the parallella: "nc -lp 12345 | pv >/dev/null"
I got about 40 MiB/s, with the Parallella being 75% busy (1 core netcat, 1/2 core pv), using the newest image on a stock configuration.