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I admittedly have never daisy chained firewire hardware on a mac, but the only obvious problem I see is that its a shared bus, so of you connect a few high speeds devices the performance of each individual device might suffer.
 
You can use RAID over daisy chained FW hard drives if you want to improve performance. Daisy chaining will not give you the same problems you're having with USB. There's really not a disadvantage unless you're trying to read/write large amounts of data to 2 or more drives on the same chain, then obviously performance will slow some because of the transfer ceiling.
 
Another question ...

If I connected a FW800 disk enclosure to the PB, then a FW400 disk enclosure to the FW800 (I have a triple-interface enclosure) does the FW800 slow down to 400 to make the daisy-chain all the same speed?
 
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