How To Configure A JBOD Disk Array (PowerVault) For Performance

We have an application that is experiencing I/O contention,
particularly in tempdb but also in two other databases. The data is
stored on mirrored PowerVault 220's, each with 10 of 14 possible disks.
The PowerVaults are JBOD devices, not true SANs. The current config has
four separate groups of physical drives assigned to distinct logical
drives for log files, tempdb, and the two app dbs. This means, for
example, that tempdb resides on one mirrored drive. The standard advice
when faced with disk contention is to add spindles if possible. With 4
empty slots, we would presumably assign the new physical disks to the
most stressed db, e.g. tempdb.

An alternative arrangement would be to combine all the physical drives
into one logical drive, and put all the files, log and data, onto the
single logical drive. The hope for this configuration is that the
PowerVault would automagically distribute the data among the drives
such that all drives were in use, all spindles reading and writing at
maximum capacity when necessary. It is my understanding that
full-featured SANs, like NetApps and EMC models, do this. My question
is whether this configuration is best for the PowerVault, as well. Or
is this the essential difference between JBOD and a true SAN?

Has anyone tried both arrangements?

Advice is much appreciated.