How fast should the volume speed be to maximize Photoshop CS5 save speed assuming one is using the DisallowFlateCompressedPSD.plugin?
My report discusses the issue for 1/2/3/4 drives.
Note that by default, all MPG Pro Workstations ship with a 2-drive RAID-0 stripe, which yields a big chunk of the improvement. For working with really big files, a 3 or 4 drive stripe is possible, but that also requires external Time Machine storage or similar protection. I personally use a 3-drive stripe for my main machine, along with an external Time Machine backup (and many other external backups).