The test that showed the greatest difference between FireWire 800 and FireWire 400 involved simultaneously reading and writing a large file to the same drive, which is what happens during file duplication. When duplicating a 1GB file, the FireWire 800 drives completed the task 38 percent faster than the FireWire 400 drive. When duplicating a folder filled with 1GB of files and subfolders, the FireWire 800 drives were 23 percent faster. When copying a file from the internal boot drive to the external FireWire drive, the FireWire 800 drives were 30 percent faster than the FireWire 400 drive. Other tests didn't show as much of a difference. The FireWire 800 drives copied a 1GB folder of files to the internal boot drive 17 percent faster than the FireWire 400 drive. Backing up the same 1GB folder from the internal drive to the FireWire 800 drives with Retrospect was a bit more than 9 percent faster than with a FireWire 400 drive.
For what I'd be doing it looks like an external Oxford bridged Firewire 800 device is really not significantly faster than Firewire 400 device -- even on Mac with Firewire 800 support.