Disappointing performance: 5-disk RAID6, 3.11.6

I have a 5-disk RAID6 using (5) 320GB SATA drives.
I rarely see even sequential I/O approaching that of a single drive's
performance.
Example: (Rarely!) I'll see an aggregate 250MB/s read or write, but
that translates to 50MB/s read or write per-drive. I was hoping for
more.

The partition layout looks like this:

/dev/sda1 : start=     2048, size=  1024000, Id=83, bootable
/dev/sda2 : start=  1026048, size=  1024000, Id=82
/dev/sda3 : start=  2050048, size=623091712, Id=fd
/dev/sda4 : start=        0, size=        0, Id= 0

on all 5 disks, and sd{whatever}3 is used to assemble the raid,
specifically, /dev/md2.

mdadm -D /dev/md2:

/dev/md2:
        Version : 1.2
  Creation Time : Fri Nov  1 11:13:07 2013
     Raid Level : raid6
     Array Size : 934242816 (890.96 GiB 956.66 GB)
  Used Dev Size : 311414272 (296.99 GiB 318.89 GB)
   Raid Devices : 5
  Total Devices : 5
    Persistence : Superblock is persistent

  Intent Bitmap : Internal

    Update Time : Tue Jan 21 10:33:52 2014
          State : active
 Active Devices : 5
Working Devices : 5
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

           Name : turnip:2  (local to host turnip)
           UUID : bece804d:eaaeb280:38d2d7f3:1e493146
         Events : 21788

    Number   Major   Minor   RaidDevice State
       0       8       51        0      active sync   /dev/sdd3
       1       8       35        1      active sync   /dev/sdc3
       2       8        3        2      active sync   /dev/sda3
       3       8       19        3      active sync   /dev/sdb3
       4       8       67        4      active sync   /dev/sde3

The filesystem is ext4, and debugfs says:

RAID stride:              16
RAID stripe width:        48


The processor is an AMD Phenom 9150e (quad-core, x86_64) and the O/S
is openSUSE 13.1, kernel 3.11.6. Some of the hardware looks like this:

00:00.0 Host bridge: Advanced Micro Devices, Inc. [AMD] RS780 Host Bridge
00:01.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] RS780/RS880 PCI
to PCI bridge (int gfx)
00:07.0 PCI bridge: Advanced Micro Devices, Inc. [AMD] RS780/RS880 PCI
to PCI bridge (PCIE port 3)
00:11.0 SATA controller: Advanced Micro Devices, Inc. [AMD/ATI]
SB7x0/SB8x0/SB9x0 SATA Controller [AHCI mode]


Settings:
The stripe_cache_size is 4096 (see
http://blog.jamponi.net/2013/12/sw-raid6-performance-influenced-by.html
)
readahead is 16384
scheduler is deadline
queue depth per-drive is 1.
nr_requests is 256.

Does this seem out of line? Thoughts?

-- 
Jon