My RAID6 8x 2TB-drive SATA XFS gives:
#dd if=/dev/zero of=/new/test bs=1M count=32768 32768+0 records in 32768+0 records out 34359738368 bytes (34 GB) copied, 152.032 s, 226 MB/s
#dd of=/dev/null if=/new/test bs=1M 32768+0 records in 32768+0 records out 34359738368 bytes (34 GB) copied, 57.7027 s, 595 MB/s (wow!!)
During the whole write time both CPU cores were 90-100%, mostly 95-99%! Glad to see RAID/XFS code is multi-core aware. For reading it was 1 core at 100% and the other around 20%. The write limiting factor appears to be my piddly Pentium D on my file server. Still, this is 3-4X the speed my old (1TB drives, crappy PCI SATA cards) array was giving me. The read is quite interesting in that the 100% CPU indicates it is probably doing parity checks on every read.
I think a big part of the good speed is my new 8-port SATA card, an Intel PCI-Express x 8 in a x8 slot. If your SCSI card is just PCI, then the PCI MB/s speed limit is what's killing you. Even PCI-X may be limiting. And the Intel card was pretty cheap, under $200.
BTW, I got stuck with two spare SATA card expander cables (1 card port to 4 SATA drives) if anyone wants some cheap. I can get in the Intel cards too, if anyone wants a complete package.