- FireWire devices are more expensive to build, because the device needs to fully have the ability to communicate rather than just naively respond to requests with chunks of data.
- FireWire does NOT offer anything that USB cannot. If your USB devices are slower, this is due to cost cutting and using cheap chips, memory and manufacturing techniques. This cost cutting is probably the main reason FireWire has not become an industry standard.
If your USB devices are slower, this is due to communication overhead, polling, and how every piece of data has to go through the CPU to get where it's going.
I've found that my USB/Firewire hard drive enclosure gives slightly worse performance and noticeably higher CPU usage when on USB. Oh, and there's also the annoyance of it taking up two USB ports that way (one for data, one for power), whereas it can power itself entirely from the single Firewire cable.
- Target Disk Mode is no longer necessary over FireWire because Ethernet can do the same thing (but potentially at gigabit speeds)... I don't know the precise details of this, but it's a completely plausible option and makes sense.
Not quite. To replicate Target Disk Mote, you'd have to add support for TCP/IP, DHCP, probably Bonjour, oh, and some sort of filesharing protocol into EFI. (Some of this is already present in the MacBook Air.)
But wait, there's more! Target Disk Mode makes the entire drive accessible to the host machine; that is, it appears as an HFS+ block device. You can reformat it, repartition it, whatever; you're directly accessing the hard drive.
For a theoretical Ethernet Disk Mode to work and to be as fully-featured, you'd need to have a special network block device driver of some sort; sharing over AFP/SMB wouldn't cut it. Oh, and the concurrency issues of having multiple people connected would be fun, too.