Performance and Mirroring

IDS 9.4 on Sun 450R, Solaris 8 ... My primary chunks are on two internal disks and mirrored to an external RAID. The RAID is about four times faster than internal disks, as measured by pfread. The reads seem to be using the mirrored chunks because the performance (for some of the selects) is at least as good the non-mirrored test database residing only on RAID. I wish I had some extra space on the internal disks; so I could mirror the RAID test database to the internal disks and test performance. Will I get a better performance if I move all my primary chunks to the RAID and mirror them to the slow internal disks?