Until the Q950 and Q840AV, SCSI performance on the Mac was uniformly pathetic and only achieved "barely" acceptable in those models. I'd think even a standard SCSI card would blow the doors off the crappy implementation of SCSI on the Mac, so it certainly can't hurt to try!

Which Mac?

Is the card Fast/Narrow SCSI II or Fast/Wide SCSI II like the JackHammer?