Hello,... We have some MacPros, and we use the inside interface from the Macs with 3 x 250 Gb SATA II RAID 0. We are pretty happy with them.
I do not know your experiences with the SCSI systems, but we here did not had very luckly, so I have no comparasion for that.
What I can say is we can achive to play with NO frame loss 30fps in our case, up to at least....

1 mov with 2400 x 800 PhotoJpeg 75%
or
1 mov with 1024 x 768 PhotoJpeg Uncompressed

2 mov with 1024 x 768 PhotoJpeg 75%

4 DV movs from the original Cat Library.

I am conservative in that, and of course depends a lot of the movie, and other factors. I do not know what you can achive with the SCSI, but for us here is doing the job, and is cheapper and easier to work than the SCSIs.