I have a Firewire 400 external hard drive enclosure I can get sustained speeds of about 35 MB/sec, the hard drive inside of the enclosure is a SATA drive, which can theoretically run at 3 Gb/sec.
I also have a USB external enclosure with the exact same SATA drive in it, and the max speed I get from it is a measly 20 MB/sec. USB's overhead is rearing its ugly head.
Also, with USB the CPU is doing most of the work since there is no DMA, whereas with Firewire you get DMA which means the CPU is doing almost no work. Having ran entire OS's off my Firewire drive, it is plenty fast for gaming and anything else you would like to accomplish with it.
There is no real lag period either, or it is so miniscule that it can't be measured. Also games tend to pre-load data into memory whenever they get a chance, and the OS will attempt to do read-ahead as well. If anything games these days are designed to eat up as much memory as possible to lessen the chance that they have to hit the hard disk for data and or using the fact that they are going to know where you are before you know it that they can preload the next few scenes.
FW800 should also be significantly faster than FW400. I don't have any FW800 enclosures or I would test for you.
Firewire was always the better standard, but more expensive (because of the way it is implemented). Had Apple not charged for Firewire implementations then maybe USB would not be here today, well other than that Apple stood behind Firewire, and Intel stood behind USB ...