iostat reports 83MB/s for each disk, running up to 830MB/s for all 10 disks,
but the max read speed of the array is approx 256MB/s.

This may be a dumb question but what motherboard & disk controllers
are you using and how are they setup?

That 256MB/s feels to me like you may be bumping up against a
bandwidth limitation of your bus. PCI (32bit/33MHz) is limited to
133MB/s max and PCIe(1.0/2.0) is limited to 250/500MB/s per lane. To
sustain 830MB/s you need at least a PCIe (1.0) x4 or PCI-X 133
controller card which are both rated for (theoretically) 1GB/s.

As an aside, for read speeds on a RAID-5, the parity disk isn't used
so theoretical maximum in this case is 9x83MB/s or 747MB/s. ;-)


-- 
Drew

"Nothing in life is to be feared. It is only to be understood."
--Marie Curie