I’m going to use the highest data rates I found for DVD writing:
1x DVD-R = 1385KBps = ~1.36MB/s
16xDVD-R = 22160KBps = ~21.64MB/s
And on firewire…
Firewire 400 = 400mbit/s = ~40MB/s (due to overhead).
Firewire 800 = 800mbit/s = ~80MB/s (due to overhead).
It’s probably not safe to run more than one 16x DVD±R drive per firewire 400 channel. Per firewire 800 channel, you can support two, maybe three.
If you back down the burn speed (if allowable with the blank media and drives chosen), you could support two maybe three with firewire 400, and six or so with firewire 800.
So, if you want to use the bridge boards that support four drives, I’d suggest sticking with 8x burning. In addition, do not daisy chain the firewire boards but, instead, connect each to a separate firewire 800.
An alternate plan would be to look into a SATA-based solution. You can find SATA->IDE(ATAPI) bridge boards pretty cheap if you look around.
I think the biggest problem you are going to discover is the limitation of the host hard drive I/O and PCI bus, and potentially IO scheduling issues in the Windows OS (in my experience, it doesn’t schedule transactions well for parallel optical IO). A more stable solution (other than using a standalone duplicator) would be: a contemporary speedy hard drive array, much more recent motherboard and an on-board or PCI-express (PCIe) SATA controller that supports multi-lane eSATA.
And don’t even think about USB.