Keep in mind that the structures in a storage pool and FAST cache are exactly as you mention: private RAID groups.  Therefore, the command you'd actually want to run is "getrg" (and not storagepool or cache -fast); however, this only lists the non-private RAID Groups.  You'll also want to pass the engineering password to expose the private RAID groups.  navisecccli -h getrg - engineering_password   This will list the RAID 1 pairs in the case of FAST Cache (lets you determine how the PRIMARIES and MIRRRORS were separated) in addition to the private RAID Groups in the storage pools.
I've heard that naviseccli can be used to display the private raid group layout underneath a storage pool but don't see the options in the latest guide.  I've tried "storagepool -list" and "storagepool -feature -info" to no avail.  Does anyone know how to do this with naviseccli or another method?  Thanks.
display private raid groups