Then you’ve never set up two devices on two identical machines and let ‘em run the same sized data package. That 480 vs 400 makes a difference over 30GB.
Unfortunately I currently don’t have two identical setups to run. One is Firewire 800 and the other USB2. It is also possible you have a cheap Firewire controller. That can impact speeds for either protocol.
That's the problem: the 480 is raw megabits per second of communications, but it has a lot more overhead than the 400, and that overhead eats into the 480. People are usually lucky to get half of that 480 in actual throughput.
I can't test here because my Firewire is a RAID at 800, while the USB is just one disk. It wouldn't be realistic.