No matter how fast your CPU is, you aren't going to beat a dedicated hardware RAID controller. Also, if you're going to spend the money for SCSI, why wouldn't you spend a little more and go with a hardware solution? That's like buying a BMW then "saving money" by adding the fog lights yourself.
You are going to beat hardware controller, because the chip running your software RAID (P4 Xeon, 2GHz) is much faster than the chip on the hardware controller (arm, 100MHz). Your only limitation is the IO bandwidth, thats why you go with SCSI.
Server manufacturers sell hardware RAID as expensive add-on, but they are not advertising any benchmarks showing speed advantage. Because there is none. Current controllers are just not good enough, can't keep up with speed advances of CPUs.