this sounds like we have more than a bit of fun ;-) ...
SCSI host adapter in a breakout box - i wouldn't do that, because those breakout boxes have limited throughput (usually 1 PCIe 1x spreads to 5 PCI slots), i'd rather consider to use a *real* PCIe or PCI-X SCSI host adapter to stick it directly in the macPro. i have also seen a IOI adapter with SCSI + FW800 on a single expansion card.
additionally: any new penryn motherboard should be able to access SAS disks (which is the TLA for Serial Attached SCSI) which would be the more modern and performant (and internal) solution to hold sample data (and finally less expensive than a SCSI host adapter + SCSI disks + enclosures + a lot of cables)
christian
and remember: only a CRAY can run an endless loop in just three seconds.