Your statement about the 3 times speed being only theoretical, but quicker is quicker. I have a fewHDD enclosures all with either USB 2.0, Firewire, SATA or eSATA, and without a doubt the USB and Firewire are close, but the eSATA knocks them in the dirt. Not sure why you are making some claim the hard drive can only read/write so fast. The throughput of a ATA133 drive still has the bottleneck of the interface, which even SATA can't surpass yet so that's a non-issue as Firewire is slower still. Your point of Firewire powering the peripherals is valid but for HDD's thats waiting for a disaster without secondary power at least.
As far as a port multiplier I am sure you are meaning an eSATA controller card. I have one with 4 ports and for now that's all I will need. Firewire is going the way of the DODO as far as I am concerned,
Whatever the interface of the hard drive, you are still left with that drive's maximum capability to read and write. Refreshing my memory of reviews, the same manufacturer's eSATA version only wrote about 20% faster than their Firewire *400* version. Looking at a newer Tom's Hardware review, I see that the eSATA had a write burst slightly faster than the Firewire 800, but the average and slowest writing was pretty bad in comparison. And the read was far slower than the Firewire 800 drives.
Not looking too good for eSATA. It looks like both the eSATA and Firewire 800 standards are capable of more bandwidth than most single drives can use anyway. IOW, no advantage for eSATA with one drive, and some disadvantages.
As far as a port multiplier I am sure you are meaning an eSATA controller card. I have one with 4 ports and for now that's all I will need.
Good. And when I get a Firewire scanner I'll just plug it to the back of the Firewire hard drive. Same with more hard drives, a printer, etc. And I didn't have to put a card in a computer, or get mad because my onboard eSATA is complete crap (as it is in many boards).