Did a quick test here with the enclosure I have. It's a "hotdrive Mobile 2.5 HDD" enclosure with USB2 and Firewire 400. Had to dig out a wall wart for the USB part, since it seems it's not pulling enough power off even two usb cords now.

Method: cat a 1,099,777,736 byte file to /dev/null, done right after the volume was mounted to avoid caching.

5400 RPM drive USB2 connector:
real 1m9.246s
user 0m0.055s
sys 0m2.564s
15.15 MB/s

5400 RPM drive Firewire 400 connector:
real 0m43.543s
user 0m0.036s
sys 0m1.979s
24.09MB/s

7200 RPM internal SATA:
real 0m33.823s
user 0m0.026s
sys 0m1.492s
31.01MB/s

This was done under OS X 10.4.8 on a MacBook Pro 15 inch, so unfortunately no Firewire 800 ports to use. I'll try it on Windows sometime if I find a good utility for doing similar testing.