SCSI drives do not really see that much of a performance gain until you get 10,000-15,000 rpm drives. the seek times on 7.2k drives just is not much better than IDE, and write is taken down because of having to go through the card and all that before writing to the drive.
That is not the main reason, but the gains just are not that great. seek times, and buffer of slower scsi drives are no better than most moden IDE drives
If you want to see real gains with scsi, buy two 10k drives with 4-5 second seem times. that is the way to get the speed you are looking for.