Um.. both yamaha and plextor make CD-R drives, not DVD. Neither make DVD drives.
I think only Sony, Toshiba, and Pioneer make SCSI DVD drives. Pioneer is the only one that makes a SCSI above 5X. They make a 10X SCSI, the 16X is IDE only. Once the next generation of IDE DVD drives from pioneer come out, they will make a 16X SCSI.
I have the 16X IDE, and kinda wish I had gotten the 10X SCSI, but at that time the 16X IDE had just come out, so the 10X SCSI wasn't availible, only the 6X SCSI was. So I got the 16X IDE. Its plenty fast, and since its the only IDE drive on that channel, it doesn't matter much anyway.