Firewire 800 appears to top out at about 83mb/sec. FW400 at about 39mb/sec. The sata drive I used for testing tops at 89mb/sec so that's where my sata connection shows, but I assume it can go a good deal faster.
subquality cables can make the ports negotiate a slower speed, the chips themselves can have their own lower limit, (VERY common with USB, but rare with firewire) and a poor cable can cause transfers to stutter if the ports are trying to transfer above the safe limit and are getting errors and retransmissions.
I've achieved up to 38mb/sec over USB2, but it's very uncommon, MOST adapters and enclosures are in the 18mb/sec neighborhood, many are 9-11, and a few are 27'ish. (most of the drop-in docks like the StarDock go as low as 6-7mb/sec) OWC's mercury enclosures always seem to hit theoretical max, they're using good hardware.
A white isight camera cable is a good example of a low quality firewire cable, it was meant to be thin, not high data rate. I don't like working with really stiff thick black firewire cables. They also take up a lot of room in my bag. So I keep an isight cable instead. It's not always the fastest, but it's convenient. If I know I'm going to be doing a large data transfer I'll grab a better cable on the way out.
All of this is dependent on the source and destination of the information. Single large files will always copy faster than many smaller files, due to overhead on both ends.
