I think this article gives an excellent overview of defragmentation of SAN drives
http://www.las-solanas.com/storage_virtualization/san_volume_defragmentation.php
Basic points is that defragmenting is not recommended on SAN storage because it is difficult to correlate the physical location of blocks on the disk when the location has been virtualized by the SAN when presenting the LUN.
If you were using RAW device mappings or you have direct access to a RAID set that is the LUN you are working with, I could see degfragmentation having a positive effect, but if you’re given a “virtual” LUN off a shared RAID-5 set, no.