It is possible that the SAN has one (or more) large pools of storage - possibly with flash, SAS, and NL-SAS (SATA) drives in the same pool with a tiering algorithm that moves recently (or frequently) accessed data to the fastest disks. Their could be several sets of RAID5 or (hopefully) RAID10 sets backing the pool(s).

So, your storage admins think that it doesn't matter, and they may be right. Still, there may be less contention at some level if you ask for separate LUNs.

Monitor your latency: http://www.sqlskills.com/blogs/paul/how-to-examine-io-subsystem-latencies-from-within-sql-server/[/url]