i would assume lower latency is due to FW800 protocol and maybe some engineering feats, AJA tightened up the IO considerably over the 4 years of development. As far as the FW 800 of the IOHD and the FW400 of the IO series, the bitrates stay the same over a larger pipe. Prores 422 HQ HD formats are merely 220megabits and as everyone is gettting to know, that equates the bitrate of full faster 10 bit high quality HD application codec with that of uncompressed SD.
SIDENOTE or maybe the point:
you are not supposed to use firewire drives on the same bus with the IO, IOHD or most other video firewire device that captures and does not transfer. you can use the PC or PCI-express 34 slot for the storage device be it firewire 400, 800 or eSata.