Unless there's some top of the line DVD-ROM that's only in SCSI interface, you won't get much benefit from a SCSI. (the same DVD-ROM will perform (almost) as good with IDE as with SCSI-interface).
Not that SCSI is all dead, but the performance margin on optical drives is just too small (except when writing to several drives at the same time from various sources). SCSI Harddrives are another case; 10-15k rpm, fast and lots of cache and some fuckin fast accestimes and durability make it still worth for a Fileserver or very HD-stressing apps.
I searched for a SCSI-DVD-ROM too some time ago, but it wasn't worth the effort and I took a 16xPioneer and a 16xASUS with IDE interface. I don't regret it