RAID 5 throughputs

Hi All,

I have a storage system with 12 Maxtor 7200 rpm 250GB hard drives in
RAID 5 configuration with one hot spare. Maxtor claims that each drive
is capable of providing 56 MB/s sustained throughput. My host system
is connected to it using 64bit ultra320 Adaptec SCSI card. The host
system has 3.0 GHz pentium 4 (with HT) processor and 4 GB of RAM
running linux kernel 2.6.9. I was wondering what order of "maximum"
sequential read/write, random read/write throughput should i expect
using them. I am using some of the benchmarking tools like Bonnie,
Bonnie++, tiobench, dd, iometer etc. However the results from them
have been very depressing. Theoretically i should be able to achieve a
read sequential throughput close to min(10*56 = 560, 320) = 320 MB/s.
However my results are no way close to it. I was wondering if anybody
can give me some insight of what order of practical throughputs i
should expect using linux and what other factors may be creating
bottlenecks.

TIA