I'm seeing almost the exact same problem. My setup is as follows.
Internet Provider: Cox
Typical Speed: 15mb down / 4mb up. Measured with http://www.speakeasy.net/speedtest/
Usenet Provider: usenetserver.com
I'm running FreeNAS-8.0.4-RELEASE-p1-x64 (11059), sabnzbd 0.6.15 and sick beard on the same machine. It is WIRED into a gigabit port on the wireless router.
If there is an active NZB downloading sabnzbd will max out at 1.6mb down. If I run a speed test during this time I get less than 1.5mb down and 4mb up. As soon as i pause the NZB download and run the speedtest it goes back to the normal 15/4 speeds.