Well, keep in mind that Mb is megaBITS, which are 1/8 the size of a megaBYTE.
You probably already knew that, but if you didn't, then it will probably be a big shocker that 400 Mb/sec only equals 50 MB/sec.
And firewire may have that kind of bandwidth, but finding a firewire DRIVE that will push data at that rate...well....it just ain't gonna happen.  I'm sure there are faster ones than my friend's, but they come at a price.  Let me put it to you this way:
He got a 30 GB firewire drive.  $350.00.  It sustains 5 MB/sec.
I got a 30 GB IBM GXP75.  $150.00  It sustains 38 MB/sec.
If you are looking for SPEED of a drive, then NO WAY IN HELL is firewire the way to go.  If you are looking for the fastest, easiest PORTABLE drive, then go that route, but NO, firewire is not gonna be the biggest performer.
Ricky
DesignDawg