Well, as it turns, besides borking the filesystems, WD also let their marketing guys have a say in the hardware configuration for packaging purposes. The box has "gigabit ethernet" printed all over it, implying gigabit data rates. Well, according to forum post the device barely exceeds 10Mb of throughput due to its lightweight embedded ARM processor:

Dear 1,

Thank you for contacting Western Digital Customer Service and Support.

Our internal testing shows that the MyBook World's will transfer at 
24-40Mbps (3-5 MBps) on a local network. The drive does not move data
quicker because that is the maximum throughput that the enclosure's
CPU can handle.

Sincerely,
Anthony M.

Western Digital Service and Support
http://support.wdc.com

In my tests of single large files (2GB) over CIFS I only see about 2.5 MB/s (20 Mbps). This is a hardware limitation I cannot overcome, and may force me to return the drive. I'll do some practical usage tests and see how badly this impacts my intended use of the device.