Most articles are discussing Storage Spaces as a virtual RAID meant to replace hardware RAID. I haven't been able to find much discussing using it in conjunction with a hardware RAID or how having mirrored disks on different storage devices might effect performance.
My thought being to have a local RAID then combine it with the software RAID to get a combination result, something similar to combining a RAID 1 and RAID 0 to create a RAID 10 - the difference here is it does not necessarily need to be on the same hardware.
So the question would be, what if locally we ran a stripped array on fast (10k, 15k, or SSD) storage then used a Storage Space to mirror that to a slower JBOD or network share (7.2k) and utilize a write-back cache.
The above could also be in conjunction with using tiered storage. The idea being to have fast storage locally that can do the heavy workloads, but have that mirrored to a secondary location without impacting performance of those local drives.