My BlizzardPPC was overclocked to 280Mhz by uping the bus speed to 70Mhz (x4 multiplier) and didn't affect scsi.
BlizzPPC SCSI uses the 68k clock, so clocking PPC has no effect on it.
However, my BPPC run at 264MHz (66.0MHz bus, 4x multiplier)+ 64.1MHz (64.1MHz bus) using SCSI.
However, with such 68k bus overclocking the SCSI timing is skewed, which can result it erratic operation. Apparantly modern HW has high enough error tolerances in timing and everything seems to work fine. It may depend on the used SCSI devices though, so if you want to have 100% reliable SCSI, don't touch the 68k clock.
Oh, the A1200 is mostly powered off now, writing this on my Pegasos 600MHz where UDMA IDE is 5x faster than the BPPC SCSI2 anyway... :-)
