yes, SAS disks are in a similar price range than formerly SCSI - they are typically used in high performance servers.
btw: the RME MADI is also available as PCI express ... and maybe you should ask someone who tried this setup, but possibly the internal routing (VE out - MADI - logic in) can be actually done in the internal RME mixer, so leaving the 64 channels (external in) free for routing audio from macPro-2 to macPro-1
look at the specs ... Three open full-length PCI Express expansion slots One PCI Express 2.0 x16 slot Two PCI Express x4 slots
looks like you could even insert 1 SCSI hostadapter + 2 x PCIe MADI (in case the brackets/wordclock modules allow)
christian
and remember: only a CRAY can run an endless loop in just three seconds.